What does Genesis 18:20 mean?

Having explained why He will reveal His plans for Sodom to Abraham, the Lord now begins to do so. Apparently, Abraham was unable to hear the Lord's words in the previous verses. Now the Lord speaks directly to him.

What the Lord says is ominous. The outcry against the peoples of the cities of Sodom and Gomorrah is great, and their sin is grave. The language used is similar to what God said to Cain in Genesis 4:10, "Your brother's blood cries out to me from the ground." In the same poetic sense, the cries of the victims of the sins of Sodom and Gomorrah had reached the ears of God Himself. He was about to execute justice.

God's reason for revealing this to Abraham, in advance, seems to have several layers. First, since Abraham is meant to be the founder of God's chosen people, he needs to see that God's promises—all of His promises—are kept. This not only means the covenant with Abraham, but God's judgment of sin and wickedness. Also, the opportunity for Abraham to plead for the innocent of Sodom, of whom there are virtually none, highlights for Abraham just how depraved that culture had become. These combine to highlight God's perfect justice, which is something He wants His chosen people to understand.