What does Genesis 16:12 mean?

In the previous verse, the angel of the Lord--perhaps Christ Himself, given the phrasing used (Genesis 16:10; Genesis 16:13)—began a prophecy or "birth oracle" about the baby Hagar is carrying. It is a boy and she is to call him Ishmael, a name referring to this moment when God has heard and helped her.

The rest of the oracle is less favorable. Even though Ishmael will be the first son born to Abram, he is not the promised child which God has guaranteed. This was not the way God intended to fulfill His vows to Abram. And so, this firstborn boy of Abram will become a wild donkey of a man. He won't be a people person. His hand will be against everyone and everyone will be against him. He will live in hostility to his kinsmen.

Eventually, we will learn that Ishmael's descendants become the Arabic people. These cultures have been at odds with the Jewish people for millennia. The descendants of Ishmael soon take on a bedouin lifestyle, on the fringes of society, committed to their personal freedom above the need to be accepted by others.