What does Genesis 15:19 mean?

In the previous verse, God established His unilateral covenant with Abram and his descendants. This is a promise which depends only on one side's agreement: in this case, God's vow, which He will fulfill no matter what Abram or his descendants do.

That covenant includes possession of the land of Canaan as defined by God Himself. Verse 18 revealed the southern and northern borders to be the "river of Egypt," meaning the Wadi el-Arish River, and the Euphrates River, respectively. 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. He begins in this verse by naming the lands of the Kenites (who lived in the south), the Kenizzites, and the Kadmonites.