What does Genesis 16:15 mean?

In obedience to the angel of the Lord ("Yahweh"), Hagar returned to Abram and Sarai. What must they have thought and felt when she reported that the Lord had found her, and explained the promises He had made to her? Her running away was triggered by harsh treatment from Sarai (Genesis 16:6), who was angered by Hagar's newfound contempt (Genesis 16:4). Whatever Sarai might be thinking at this moment, her relationship with Hagar will, it seems, never become warm. In fact, in chapter 21, we will see Sarai—by then renamed Sarah—demand that Abram cast Hagar and her son out of the family (Genesis 21:9–10).

In any case, Abram does indeed name the boy Ishmael, which means "God hears." The name certainly has meaning for Hagar (Genesis 16:10–13). And, it might have also sent a message to Abram and Sarai. Through their experiment attempting to bring about God's promise on their own timeline, they have learned He was still listening, still seeing, and would still keep His promises to Abram.

In any case, it's clear both that God was acting in the birth of Ishmael and that, though this son would be blessed, he was not the child of God's covenant promises to Abram.