What does Genesis 21:18 mean?

Hagar and her son Ishmael have come to the end of their lives, as far as she is concerned. They are out of water and lost in the wilderness. She has recently put him under a bush and sat down some distance from him, expecting him to die. Then an angel of God called to her from heaven. God had heard Ishmael's voice.

Now the angel instructs Hagar to get up and move back to her son, to lift him up and embrace him. In other words, it's not time to give up. The angel renews God's promise that Ishmael would become a great nation. This is the only reason Abraham agreed to Sarah's demand that Hagar and Ishmael be cast out: God's promise to prosper Ishmael, not to destroy him (Genesis 21:12–13).

A common theme of the book of Genesis is that God's plans are often fulfilled in ways we would not have expected. They sometimes take longer to complete than we'd prefer. Hagar's story serves to remind us that our individual stories aren't done until God's promises to us have all been fulfilled, no matter how bleak things look in the moment.