What does Genesis 18:10 mean?
Abraham has been entertaining three strangers who arrived mysteriously outside of his tent. He hurried to provide a royal feast for them. It's unclear whether Abraham understood clearly that one of the men was the Lord in human form before this moment, but now the Lord will reveal Himself completely. In the prior verse, the visitors asked where Sarah was. Knowing that these men are God and two angels, this question is really for Abraham's benefit, not their own.Here, God speaks to Abraham about Sarah with the full knowledge that she is listening. The Lord repeats what He has said to Abraham in the previous chapter. This time, though, He speaks in a way that Sarah will be sure to hear. God announces, and Sarah clearly hears, that she will have a son by this time next year, when the Lord plans a return visit. Hearing such an announcement would clearly confirm for Abraham that this was the Lord who had made the same promise to him in the previous chapter.
Sarah, though, seems to be hearing this promise for the first time. Most likely, Abraham had not told her what God had revealed to him in the prior encounter. Sarah's initial reaction will be similar to Abraham's. Given her lifelong barrenness and advanced age, she will laugh at the suggestion of having a natural-born child.