In God vs. Cast off from God
Undoutedly, St. Paul is saying that a certain number (not necessarily all) of the Jews were "cut off" because of their unbelief.
What is it to be 'cut off'
"Cut off" can be a Hebraism for literally killed, or cast off from something you possess or are to be the recipient of.
Here, it means the heavenly inheritance (ultimate salvation and the promises to those who are to recieve it) as "the Israel of God" (Gal 6:16):
The natural branches refers to those grafted into God 'originally,' i.e. the Jews. 'You' here refers to the Gentiles, to whom he writes.
Since the natural branches did not accept the Christ, they are "not..[spared]." Thus, St. Paul warns those who have newly accepted Christ and been grafted in 'unnaturally' (that is, not being Israel literally, but spiritually, grafted in on account of Christ and being His Body, and on His account recieving the promises) not to fall into the same unbelieve, otherwise they too will be "cut off." There is no way around this, it means you can lose your salvation or be 'ungrafted' off from the branch just as easily as you were grafted in—if it can happen to the natural branches, much more so us, the 'unnatural' branches:
Thou wilt say then: The branches were broken off, that I might be
grafted in. [You say] well: because of unbelief they were broken off. But
thou standest by faith: be not highminded, but fear. For if God
hath not spared the natural branches, fear lest perhaps he also spare
not thee. See then the goodness and the severity of God: towards
them indeed that are fallen, the severity; but towards thee, the
goodness of God, if thou abide in goodness, otherwise thou also shalt
be cut off. And they also, if they abide not still in unbelief,
shall be grafted in: for God is able to graft them in again. For if
thou wert cut out of the wild olive tree, which is natural to thee;
and, contrary to nature, were grafted into the good olive tree; how
much more shall they that are the natural branches, be grafted into
their own olive tree?
St. Paul clearly teaches that the Jews became "cut off" (meaning they were once part of the original 'vine') and that we must be fearful lest the same happen to us.
Jesus teaches precisely this in His parable (one of the more explicit ones) about Him being the Vine into which we are actually grafted:
I am the true vine; and my Father is the husbandman. Every branch in me, that beareth not fruit, he will take away: and every one that beareth fruit, he will purge it, that it may bring forth more fruit. Now you are clean by reason of the word, which I have spoken to you. Abide in me, and I in you. As the branch cannot bear fruit of itself, unless it abide in the vine, so neither can you, unless you abide in me. I am the vine; you the branches: he that abideth in me, and I in him, the same beareth much fruit: for without me you can do nothing. If any one abide* not in me, he shall be cast forth as a branch, and shall wither, and they shall gather him up, and cast him into the fire, and he burneth. If you abide in me, and my words abide in you, you shall ask whatever you will, and it shall be done unto you. In this is my Father glorified; that you bring forth very much fruit, and become my disciples. As the Father hath loved me, I also have loved you. Abide in my love. If you keep my commandments, you shall abide in my love; as I also have kept my Father's commandments, and do abide in his love.
* An older way of saying 'remain': Greek μενη—'does remain'