What does Genesis 15:20 mean?

In the previous verses, God established the southern and northern borders of the Promised Land to be the Wadi el-Arish River, referred to here as the "river of Egypt" the Euphrates, respectively. That covenant includes possession of the land, based solely on God's work, and not on the efforts of Abram or his descendants.

Now the Lord continues to define the areas of the nation His people will possess in terms of the people groups occupying those lands previously. This verse lists the Hittites and Perizzites, both well-known occupiers of the land of Canaan, as well as the Rephaim, one of the people groups defeated by the four kings of the east in the previous chapter (Genesis 14:5).