Do not add to his words,
lest he rebuke you and you be found a liar.
The context of this simple warning is the veracity of scripture. "Every word of God proves true" (Proverbs 30:5). Scripture is sure, sufficient, and safe because it is the Word of a perfect, holy God Who has given us His truth. We can believe Him and live or we can disbelieve and die, which is really Adam's original sin.
But when we believe God's Word as true, we must still guard against some sinful tendencies we have in receiving it. The first tendency is to take away from scripture. We can diminish scripture. Liberal theologians have done it for generations by dismissing it as "myth" or simplistic belief belonging to a primitive time. Or they discredit prophecy by claiming it was written after prophetic events or to explain events and not prepare for them. That is the danger of making less of the Word of God. Back to the original sin: Satan did this with a question to Adam and Eve: "Did God say...?"
Yet this proverb warns us of the danger of adding to scripture. This was what Satan heard back from Even, "...neither shall you touch it..." an addition to the command that made it easier to believe the lie that Satan offered. When we add to scripture, we become liars. We attribute to God what He did not say, and we are subject to His rebuke. God should be taken at His Word. Let's not subtract from it, and let's not add to it. Let's simply accept God's truth and obey it.