God will change their names: Abram will become Abraham and Sarai will become Sarah. Abraham and Sarah will have a child. The miracle does not involve any violation of nature’s laws or any abrogation of an ancient decree; the miracle takes place through language.
Toward the end of this week's Torah reading, when God repeats his promise that Abraham will have a son with his wife Sarah, Abraham responds, "Oh that Ishmael might live before Thee!" (Genesis 17:18). It is only here, now, in these words after repeated promises, that Abraham's frustration is revealed.