What does Genesis 18:19 mean?

The Lord continues to explain why He will reveal to Abraham His plans for Sodom. In the previous verse, the Lord repeated that Abraham would become a great and mighty nation, through his offspring, and that all of the nations of the world would be blessed through Abraham.

Now the Lord adds that He has chosen Abraham, in part, to command his children, and through them, all of the children who would follow, to keep the Lord's way by doing what was right and just. In that way, the Lord would keep His covenant promises to Abraham. This places an important context on God's decision to tell Abraham, in advance, what was about to happen to Sodom. As Abraham pleads for God to spare the innocent of Sodom—of whom there are virtually none—and sees God's destruction, he will be reminded of the holiness to which he is being called.

This is the first mention of Abraham's responsibility to command and train each following generation to keep the way of the Lord. By extension, this same command is applied to Israel. This is also one of the few mentions that God's keeping of His promises to Abraham will be tied to Israel keeping to the way of the Lord.