What does Genesis 18:13 mean?

Sarah is listening from a nearby tent as her husband Abraham converses with three strangers. These men are actually God and two angels disguised in human form. The strict approach to hospitality practiced in that era would have applied to every aspect of this event. In part, that probably meant Sarah should not have been eavesdropping. It certainly would have meant that laughing at something a guest said, unless it was intended as a joke, could have been seen as impolite.

Since the details of this passage are brief, we don't know if Abraham and the three men could hear Sarah laugh in response to the idea of her having a baby. It is possible that God's divine awareness that she laughed to herself served as evidence that this was, indeed, the Lord in human form outside of her tent. The text itself gives us no way to know for sure.

In either case, the Lord asks Abraham why Sarah had laughed in unbelief about the idea. This would have been an awkward question for Abraham to answer, since his response to this same news was similar (Genesis 17:17).