The instruction is repeated almost word for word at 24:29.
Quick answer
Do not become the one who settles the score. The verse takes the case out of your hands and does not ask you to pretend nothing happened.
Read both lines
The prohibition is aimed at a sentence rather than at a feeling, which is worth noticing: it forbids saying it, the moment the intention becomes a plan. The verse does not tell the reader that the evil was imaginary or that the anger is wrong. It grants the wrong and reassigns the response. Then the second line supplies what makes that bearable, and it is not stoicism. It is wait, plus a person to wait for, and the last phrase says He will save. Not that He will punish, which is what a revenge verse would promise, but that He will get the wronged party out. The book says the same thing again at 24:29, which suggests the temptation was assumed to be constant.
What it does not mean
Waiting for the LORD is not a command to stay in danger, and this verse has been used that way. Declining personal revenge is not the same as declining protection, refusing to report a crime, or staying under someone's control while they keep doing harm. Scripture puts civil justice in a different category than private vengeance entirely, which is why Proverbs can forbid settling your own score and also condemn courts that fail the vulnerable (13:23, 19:28). If you are being harmed, getting to safety is not revenge. The verse addresses the impulse to repay, not the responsibility to be safe.
From this side of the cross
Paul lands on the identical ground and quotes the Old Testament to do it: "do not avenge yourselves, but rather give place to wrath; for it is written, 'Vengeance is Mine, I will repay,' says the Lord" (Romans 12:19). He then goes further than the proverb, telling the believer to feed the enemy and to overcome evil with good (12:20–21). The proverb takes the weapon out of your hand. The gospel puts something else in it.
Related: Proverbs 24:29 · Romans 12:17–21 · Proverbs 25:21–22 · Proverbs 19:28
New King James Version®, Copyright © 1982, Thomas Nelson. All rights reserved. Full copyright information. Scripture provided by API.Bible.
Go Deeper
If this verse has been used to put weight on you that it was never carrying, ask about it directly.