Abraham and Sarah are old; and they have no children. In order to secure some kind of future for himself after his death, Abraham decides that Eliezer of Damascus, a slave born while his mother (and father?) were owned by Abraham, shall be his heir. What else can he do?
But God has something bigger and more unbelievable in mind. God will indeed give Abraham and Sarah a biological child in their old age. What human beings cannot accomplish on their own, God can bring to pass. But the promise includes more than a direct heir for Abraham. Through this child God will give to Abraham and Sarah so many descendants over time that they will seem as vast in number as the stars in the heavens. It would be a miracle enough for Abraham to have a son in his old age, but God is not content to stop there. Abraham and Sarah will become parents of a great nation.