What is an Afikoman?
Literally - Dessert.
At the beginning of the seder, the middle of three matzot (plural for matzah) is broken with the larger piece tucked away for later - this piece is called The Afikoman.
Children over the years have been kept awake during the seder by playing a game of hide-and-go-seek with this "dessert." The person leading the seder will hide the Afikoman and at the appropriate time ask all the kids to go find it. Since the seder cannot come to a conclusion without eating this Afikoman, children will get toys or other gifts as a reward for finding it.