What does Genesis 6:12 mean?

This verse describes the extent of the corruption of sin on the earth: It was everywhere. It was everyone. And it was self-inflicted. "All people" or "all flesh" had corrupted their ways. In other words, every person was guilty of taking the good thing that God had made—including themselves—and turning it away from God's good intent to use it for selfishness and evil.

The prior verse described man's culture as "violent," from the Hebrew term hā'mās'. This not only includes physical aggression, but cruelty and injustice, as well. Man has rejected God, and turned against each other. The fact that this evil is so widespread is a primary reason for God's chosen instrument of destruction: the flood. Left alone, humanity is doomed to fall further and further into sin and evil. Without intervention, there will soon be no godly people left.

To correct the situation, God has chosen to limit the lifespan of human beings (Genesis 6:3), and will eliminate all but a tiny fraction (Genesis 6:8) in order to "reboot" the human race.