What does Genesis 15:13 mean?

The Lord will soon complete a covenant ritual between Himself and Abram, a ritual that will specify, in part, the boundaries of future Israel to be occupied and possessed by Abram's descendants.

First, however, God will reveal to Abram a prophecy about the difficult future his descendants will face before they occupy the Promised Land. Abram's offspring, God says, will be strangers, sojourners, serving others in a land that is not their own. They will be afflicted or mistreated for 400 years. God is referring to Israel's slavery in Egypt, after the death of Joseph (Genesis 50:26) and before the story of Moses and the Exodus (Exodus 1:1–8). As is common in all forms of literature, the reference to time here is a generic, round number. It is perhaps meant to refer to four generations that will come and go during that time. Exodus 12:40 and Galatians 3:17 specify the length of that captivity as 430 years.

God is making clear to Abram that, though the promise of the land will be kept, it will be kept in God's own time, centuries in the future.