What does Genesis 15:18 mean?
With the covenant ritual between the Lord and Abram completed, God gets very specific about the boundaries of the land He is promising to Abram and his descendants.Covenants between God and humans are significant, of course. Often they include conditions from God which, if met by the people involved, will result in God keeping His end of the agreement. This covenant is different. Sometimes referred to as the Abrahamic Covenant, this was an agreement in which all the conditions and promises were on God's side. For example, in the prior passage, God symbolically passes between the severed halves of the animals. This might have been a common ritual of that era, where both parties declared their obligations by walking through the middle of the carcasses. Notably, only God is shown to do this in the preceding verses—Abram's work is not part of this promise.
God was binding Himself to do as He promised no matter what Abram or Abram's descendants did or did not do. Put another way, this promise from God to the people of Israel to give them this land, was a unilateral covenant.
In defining this Promised Land, God begins at the southern border with the "river of Egypt," which many scholars identify as the Wadi el-Arish River, not the Nile. The northern border would be the great Euphrates River. The following verses will define the remaining areas of the land promised to Abram's offspring in terms of the people groups occupying those lands previously.