What does Genesis 18:17 mean?

Four "men" stand looking at the city of Sodom in the distance from a high vantage point. One is Abraham. One is the Lord in human form. The other two are angels, traveling companions of the Lord on this mysterious journey. By this time, Sodom has already gained a reputation for wickedness (Genesis 13:13).

This verse is another example of God using human actions, words, or speech to help us understand an idea. God called out to Adam in the garden, even though He knew where he was hiding (Genesis 3:9). God speaks as a man to Abraham, even though He already knows how the conversation will proceed (Genesis 18:1–3). Here, God poses a rhetorical question which helps explain why, exactly, God is going through this process of appearing to Abraham and discussing Sodom. In short, God plans to make Abraham aware so he will be all the more convinced of God's power and sovereignty. This patient process will also emphasize the extent to which Sodom had fallen.