What does Genesis 15:16 mean?

Finally, God concludes his prophecy about Abram's future family. In the previous verses, God revealed that they would be captives, serving another nation for around 400 years, before leaving that country with great possessions. Then, and not before, Abram's descendants would return to the land of Canaan "in the fourth generation." Later passages of Scripture will give a more specific number than this round figure: Israel will be in Egypt for 430 years, all told (Exodus 12:40).

God's given reason for that delay is that the sin—the "iniquity"—of the Amorite people had not yet reached its full measure, or was not yet complete. In other words, one purpose of Abram's future family, the nation of Israel, is to serve as an instrument of judgment on the Amorite people for their sins against God. However, God would not preemptively judge the Amorites or any other people group of Canaan. In His justice, He would wait for them to earn the judgment He would pour out on them through His people Israel when they came to claim the land of Canaan as their own. This delay also serves as an expression of God's mercy, allowing that much more time for the wicked inhabitants of Canaan to see their sin and repent.