Essentially, it means the cord of life. Strong's Heb. 2256, "chebel" or band. Brown-Driver-Brigs - cord, and references Ecc. 12:6 as cord of life.
The NIV does a fair job with verse 6 as:
"Remember him—before the silver cord is severed, and the golden bowl is broken; before the pitcher is shattered at the spring, and the wheel broken at the well,"
A brief summary from Adam Clarke's commentary as:
The context is aging, approaching death, and the call to remember the creator acknowledged in your youth. Verses 3 - 6 compare the aging body to a house where the keepers are the hands that become paralytic and shaky; the strong men are legs that begin to bow and no longer hold up the body; the grinders are teeth that have decayed with age, and can no longer chew strong meat; the windows are the eyes that dim and can no longer see well; the doors being shut in the streets are the lips and mouth which can no longer swallow; and the sound of grinding low is the lost teeth, and inability to chew properly; rising up at the sound of a bird is the inability to sleep soundly being disturbed at the least sound; the daughters of music being brought low is the weak and feeble voice of old age.
The silver cord is the nervous system breaking; the golden bowl breaking representing the brain and loss of memory; the pitcher broken at the fountain and the wheel broken at the cistern is the heart slowing and failing to pump; which then results in the death of the body. See full commentary here
E.W. Bullinger's Companion Bible Notes on Eccl. 12:4-6,
the doors = the openings: i.e. the mouth and ears.
streets = street (singular)
sound of the grinding is low: i.e. the mastication with gums instead of teeth is low.
rise up = start: referring to insomnia.
the daughters of musick: i.e. songs, &c, the product of music.
afraid: i.e. of ascending heights.
high = lofty, elevated.
fears shall be in the way: i.e. apprehensions of danger in journeying.
almond tree shall flourish: i.e. grey hairs shall grow scanty, or drop off, not "almond nuts be rejected"; for the teeth and eating have already been dealt with in Ecclesiastes 12:3.
grasshopper, or locust. shall be a burden = shall become burdensome: i.e. as to weight.
desire shall fail. "Desire" = Hebrew = the caperberry. Here the Authorized Version beautifully renders the figure of speech (as a version should do), while the Revised Version renders it literally (as a translation too often does). The Figure of speech is Metalepsis: i.e. a double Metonymy (App-6), by which (1) the "caperberry" is put for the condiment made from it, and then (2) thecondiment is put for the appetite produced by it. And further, since, because of its shape, as well as from the notion that it was supposed to create sexual desire, all that is intended by the figure is included in the rendering "desire shall fail".
man. Hebrew. "ddam (with Art). App-14. See note on Ecclesiastes 1:13.
Or, &c. New figures now (in Ecclesiastes 12:6) introduced, referring to the arrival (Structure, above) of death itself.
the silver cord: i.e. the spinal cord.
the golden bowl: i.e. the head, or skull.
pitcher: the failure of the heart.
the wheel. On which the bucket is brought up by a rope from the cistern, or well." Source: here
From Jamieson-Faussett-Brown Commentary on Ecc. 12:6,
"6. A double image to represent death, as in Ec 12:1-5, old age: (1) A lamp of frail material, but gilded over, often in the East hung from roofs by a cord of silk and silver interwoven; as the lamp is dashed down and broken, when the cord breaks, so man at death; the golden bowl of the lamp answers to the skull, which, from the vital preciousness of its contents, may be called "golden"; "the silver cord" is the spinal marrow, which is white and precious as silver, and is attached to the brain. (2) A fountain, from which water is drawn by a pitcher let down by a rope wound round a wheel; as, when the pitcher and wheel are broken, water can no more be drawn, so life ceases when the vital energies are gone. The "fountain" may mean the right ventricle of the heart; the "cistern," the left; the pitcher, the veins; the wheel, the aorta, or great artery [Smith]. The circulation of the blood, whether known or not to Solomon, seems to be implied in the language put by the Holy Ghost into his mouth. This gloomy picture of old age applies to those who have not "remembered their Creator in youth." They have none of the consolations of God, which they might have obtained in youth; it is now too late to seek them. A good old age is a blessing to the godly (Ge 15:15; Job 5:26; Pr 16:31; 20:29)." Source: here
And, Gill's Exposition on Eccl. 12:6,
"Or ever the silver cord be loosed,.... As the above are the symptoms and infirmities of old age; these in this verse are the immediate symptoms of death, or what attend it, or certainly issue in it. Some by "the silver cord" understand the string of the tongue; and to this purpose is the Targum,
"before thy tongue is dumb from speaking;''
and it is observed (q) in favour of this sense, that the failing of the tongue is no fallacious sign of death, of which there is no mention at all in this account, unless here; and the tongue may not unfitly be called a "cord", both from the notation of the word because it binds, and because it scourges like a cord, Job 5:21; and is compared to silver, Proverbs 10:20, and in this verse rather the head than the back is treated of. But best, the bond of union between soul and body is meant: the Midrash and Jarchi, and the Jewish writers in general, interpret it of the "spina dorsi", or backbone; or rather of the marrow of it, which descends like a cord from the brain through the neck, and down the backbone to the bottom of it; from whence spring the nerves, fibres, tendons, and filaments of the body, on which the life of it much depends: this spinal marrow may be called a "cord" for the length of it, as well as what arise from it; and a silver cord, from the colour of it (r), this being white even after death; and for the excellency of it: and this may be said to be "loosened" when there is a solution of the nerves, or marrow; upon which a paralysis, or palsy, follows, and is often the immediate forerunner of death;
or the golden bowl be broken; the Targum renders it the top of the head; and the Midrash interprets it the skull, and very rightly; or rather the inward membrane of the skull, which contains the brain, called the "pia mater", or "meninx", is intended, said to be a bowl, from the form of it; a "golden" one, because of the preciousness of it, and the excellent liquor of life it contains, as also because of its colour; now when this "runs back", as the word (s) signifies, dries, shrinks up, and breaks, it puts a stop to all animal motion, and hence death;
or the pitcher be broken at the fountain; not the gall at the liver, as the Targum, which the ancients took to be the fountain of blood; but by the "fountain" is meant the heart, the fountain of life, which has two cavities, one on the right side, the other on the left, from whence come the veins and arteries, which carry the blood through the whole body; and here particularly it signifies the right ventricle of the heart, the spring and original of the veins, which are the pitcher that receives the blood and transmits it to the several parts of the body; but when thee are broke to shivers, as the word (t) signifies, or cease from doing their office, the blood stagnates in them, and death follows;
or the wheel broken at the cistern; which is the left ventricle of the heart, which by its "diastole" receives the blood brought to it through the lungs, as a cistern receives water into it; where staying a while in its "systole", it passes it into the great artery annexed to it; which is the wheel or instrument of rotation, which, together with all the instruments of pulsation, cause the circulation of the blood, found out in the last age by our countryman Dr. Harvey; but it seems by this it was well known by Solomon; now, whenever this wheel is broken, the pulse stops, the blood ceases to circulate, and death follows. For this interpretation of the several preceding passages, as I owe much to the Jewish writers, so to Rambachius and Patrick on these passages, and to Witsius's "Miscellanies", and especially to our countryman Dr. Smith, in his "Portrait of Old Age", a book worthy to be read on this subject; and there are various observations in the Talmud (u) agreeable hereunto.
(q) Vid. Castel. Lexic. Hept. col. 3662. (r) Vid. Waser. de Num. Heb. l. 1. c. 13. (s) "recurrat", V. L. "excurrit", Junius & Tremellius. (t) (u) T. Bab. Sabbat, fol. 151. 2. & 152. 1."
Sources: here Bold emphasis mine.