The afikoman is the half of a broken matzah set aside early in the Passover seder and eaten at the very end of the meal, and its role is to hold the seder together from beginning to end.
Broken, hidden, sought
Near the start of the seder the middle of three matzot is broken, and the larger piece is set aside as the afikoman to be eaten last of all, so that nothing is eaten after it and the taste of the unleavened bread is the final taste of the night. In a widespread custom the afikoman is hidden and the children search for it, or take and ransom it, a game built into the ritual to keep the young engaged through a long evening, so the piece of bread carries both a solemn function and a playful one.
The last taste
That nothing is eaten after the afikoman gives it a particular weight: it stands in the place once held by the paschal offering, whose meat was to be the last thing eaten, and its final position ensures that the seder ends on the unleavened bread that is the sign of the deliverance rather than on the ordinary food of the meal. The setting-aside of the afikoman at the start and its eating at the close thus frame the whole seder, and the search for it draws the children into the observance, so a simple half of matzah serves at once to shape the ritual, to recall the offering, and to hold the attention of the next generation the seder exists to teach.