Part of Agur's confession, Proverbs 30:1–6.
Quick answer
Do not improve on what God said. The danger named is not that you will be punished for a technical violation but that you will end up misrepresenting Him and be shown to have done it.
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Agur's confession closes here. He has no wisdom, he cannot name God, God's word is tested, and therefore this: leave it alone.
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A new voice takes over the book here, and almost everything about him is uncertain. Agur son of Jakeh is named nowhere else in scripture. His father is unknown. The two words the NKJV reads as the names Ithiel and Ucal can be divided differently in the Hebrew consonants, which is why English versions differ so widely at verse 1, and the header word הַמַּשָּׂא can be read as a place name or as the ordinary word for an oracle. He may not have been an Israelite at all. The book of Proverbs, which has spent twenty-nine chapters teaching the fear of the LORD, hands six verses to a man whose credentials cannot be checked.
Then he opens by saying he has none. Verses 2 and 3 are not modesty. They are a denial: he did not learn wisdom, he does not have the knowledge of the Holy One, he is duller than any man. And the phrase he uses in verse 3 is the exact phrase 9:10 used for what wisdom hands you. He is standing in the wisdom book saying he does not have the wisdom book's prize.
Verse 4 explains why. Six questions, all rhetorical, all with the same answer: nobody here. Who went up to heaven and came back down, who held the wind, who tied up the water, who set the edges of the earth. The questions are Job's questions from the whirlwind and Isaiah's questions in chapter 40, and they arrive at the same place. Then the last question turns strange, asking not only for God's name but for His son's, and daring the reader to supply either.
And having established that he knows nothing, Agur says the one thing he does know. Verse 5: God's own speech has been tested and it holds. Verse 6: so do not put words in His mouth. That is the entire confession. A man with no credentials, no training and no claim ends up in the only position that was ever safe, which is holding on to what God actually said and refusing to improve it. The wisdom book's most humiliating opening produces its plainest doctrine of scripture, and the order is not an accident.
What this verse contributes
The conclusion, and the reason the previous verse was stated first. Because the word came out of the fire unchanged, anything you add to it is by definition not tested. Moses says the same thing twice in Deuteronomy, at 4:2 and 12:32, and Revelation says it at the other end of the canon. Agur's version is the sharpest of them, because it names the outcome as being found a liar rather than as incurring a penalty. The addition is a claim about God, and false claims about God eventually meet the God they were about.
What it does not mean
This is not a verse about which Bible translation someone uses, and it is not a lever in an argument about doctrine. Both uses are common and both smuggle in the assumption that the person quoting it has the unadded version and their opponent does not. The verse addresses the speaker, not the opponent. The practical form of it is quieter and harder: when God has not spoken to a situation, say so, rather than supplying a certainty He did not give. Every "God told me" that was actually a preference, every promise built out of a verse that does not carry it, every rule presented as His that He never issued, is what this verse is about.
From this side of the cross
The heaviest additions are almost always requirements. A gospel with conditions attached is the clearest case of adding to His words, because it takes something finished and says it needs one more thing. Verse 5 said the word is a shield; verse 6 says do not put anything in front of the shield and call it part of the shield.
Related: Deuteronomy 4:2 · Deuteronomy 12:32 · Revelation 22:18–19
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.