What does Genesis 21:3 mean?

Just as God had instructed them (Genesis 17:9), Abraham named his son Isaac, which means "he laughs." The repetition of the phrases here—"the son who was born to him, whom Sarah bore him"—is meant to continue to slow the reader down and force us to ponder what has happened. Two primary points stand out.

First, of course, God kept His promise. He is faithful. He said this would happen, and it did.

Second, Abraham and Sarah were old and well beyond conceiving and bearing children. Further, Sarah had been barren for her entire life up to this point. Genesis intends for us to fully appreciate the idea that Isaac is a miracle baby. A normal married couple might reasonably have ten children, or even more, during their childbearing years, leading to numerous offspring for generations to come. This particular couple, though, through whom would come the nation of Israel, had just one son under miraculous circumstances. There should be no mistake that this birth was anything but the very work of God in the lives of His people.