What does Genesis 15:1 mean?

Sometime after the dramatic events of the previous chapter, the Lord came and spoke to Abram again. Specifically, we're told that the word of the Lord came to Abram in a vision. This language is sometimes used in the Bible to describe God's relationship with a prophet. As with other prophetic visions, this encounter will involve dramatic imagery.

God's first words to Abram are reassurance: Don't be afraid. I am your shield. Your reward will be great. The following verses make it clear that Abram did indeed have questions about how God would keep the enormous promises He had made to Abram. God addresses Abram's emotions: It is safe to set aside your fear. I will serve as your shield, your protection against harm. The reward will be worth the wait.

Given that Abram had been called by God at the age of seventy-five (Genesis 12:4), and had not yet had any children, his concern is understandable. Abram is not necessarily questioning God's ability, but he is asking God for more details on how God plans to accomplish His purposes.