“having Bethel on the west and Ai on the east.” Bethel is slightly more than two miles to the west (actually somewhat northwest) of Ai, and Abraham pitched his tent and built an altar just to the east of Bethel and thus just west of Ai. Many years later, Joshua would stage his attack on Ai from almost exactly the same spot (Josh. 8:9). The locations of both Bethel and Ai may not be exactly known and are disputed by some archaeologists (although there is mainly agreement about the cities, especially Ai), but even if we today are not exactly sure where they are, Abraham did and so did Joshua, and Abraham’s camp and Joshua’s troops were in basically the same area.
“He built an altar to Yahweh there.” In Genesis 12:7, Abram built an altar at Shechem, and here in Genesis 12:8, he builds a second altar between Bethel and Ai. Abraham’s building these altars at Shechem and between Bethel and Ai was more than a simple act of worship. We know that fact because Abram traveled through much of the land but only built altars in a few places. The fact that Abraham took the time to build an altar of sacrifice to Yahweh at Shechem and also between Bethel and Ai in the heart of the Promised Land was an act of trust that God would make good on His promise that he had given the land to Abraham, even though Abraham was still a wandering sheik, traveling with herds and flocks.
“called on the name of Yahweh.” The phrase “call on the name of Yahweh” refers to prayer and supplication. Abraham and others called on Yahweh to get His help and support in life just as we do. It was certainly appropriate at this time, just as Abraham was entering the land and not knowing exactly what he would be facing, that He prayed to Yahweh for guidance and support.