The verse is suggesting that we have a choice as to how the power of the tongue is used. It can be used for either good or evil. The fruit will follow its usage.
Verse 20 can help understand verse 21.
20 A man’s belly shall be satisfied with the fruit of his mouth; and
with the increase of his lips shall he be filled. 21 Death and life
are in the power of the tongue: and they that love it shall eat the
These two verses are connected. Verse 20 states that a man’s belly (or what results a man reaps) will coincide with the fruit of his lips. A man will be filled with whatever fruit comes from his lips.
Verse 21 just intensifies verse 21 extending the power of the tongue to life and death. Those that love the tongue’s power (used for good or evil) shall eat its fruit or live with the consequences.
The word YAD (Power) in verse 21, can also mean direction. That is a good understanding of the usage here. The choice is ours, what “direction” or in what way will we use our tongue.
James chapter 3 speaks to this exact topic. James states that the tongue is a fire a world of iniquity. It is so powerful that it can change the “course of nature” or the “pathway of our lives”. He goes on to say that out of the mouth of man, comes both blessing and cursing, for with the same tongue we bless God and curse man.
James 3: 6-10: (KJV)
6 And the tongue is a fire, a world of iniquity: so is the tongue
among our members, that it defileth the whole body, and setteth on
fire the course of nature; and it is set on fire of hell. 7 For every
kind of beasts, and of birds, and of serpents, and of things in the
sea, is tamed, and hath been tamed of mankind: 8 but the tongue can no
man tame; it is an unruly evil, full of deadly poison. 9 Therewith
bless we God, even the Father; and therewith curse we men, which are
made after the similitude of God. 10 Out of the same mouth proceedeth
blessing and cursing. My brethren, these things ought not so to be.
So, ultimately the choice is ours, will we use our tongues to speak good of those around us or will we use our tongue to speak evil.