What does 2 Peter 3:6 mean?

In the previous verses, Peter has described a false argument made by deceivers. Namely, that too much time has passed since Jesus promised to come back. Therefore, they claim, He isn't coming. There is no judgment. God will not intervene in the physical world. Peter indicated that these teachers are purposely forgetting that God made the world in the first place. The One who designed the universe can alter its course whenever He decides to.

Now Peter reminds his readers that these false teachers are also forgetting a huge moment in history. There have already been times, in the physical world, when God intervened in order to bring judgment. As recorded in Genesis 6—9, God brought the great flood and destroyed nearly all of the life He had created. In other words, God has already done, in the past, exactly what the false teachers were saying He will not do in the future.

In our day, many dismiss the idea of Noah's flood, calling it a myth, a fable, or an exaggeration. Even some religious teachers are eager to diminish that cataclysmic event. Peter clearly believed in and taught the reality of Noah's flood. Jesus did also (Matthew 24:37).

Peter's whole point is that God is ready and willing to disrupt the natural course of the world as it suits Him, including using the natural world He created to bring judgment on the sins of humanity.