What does 2 Peter 2:4 mean?

Peter has just affirmed that the false teachers in the church will, ultimately, be condemned and destroyed by God. Here, he begins a series of "if" statements to make the case that God does indeed bring judgment upon those who rebel against Him.

In this verse, he begins with the angels who sinned. Peter may be referring to the events described in Genesis 6:1–4, during which the "sons of God" took human wives. Some understand this to mean that it was angels who inter-married with human women in rebellion against God and were punished. Others suggest Peter means to describe angels who were cast from heaven along with Lucifer, the devil, when he rebelled.

In either case, God did not let this rebellion stand. He judged and punished these fallen angels. Specifically, Peter uses the Greek word "Tartarus," which was a part of the Greek underworld. This is somewhat different from the other biblical references to "hell," though it shares a close relation. Peter says that these angels remain there, being held in chains, or pits, of darkness until God is ready to fully judge them for their sin.