They break the matzo in two, returning the smaller piece to the stack and putting aside the larger piece to be eaten later during Tzafun ("Hidden", the twelfth part of the Seder,  which immediately follows the main meal). This is the afikoman, which is wrapped in a napkin before being hidden.
They split the matzo into two pieces, setting aside the smaller one to be eaten later during Tzafun ("Hidden"), while keeping the larger piece as the afikoman, which is wrapped in a napkin before being concealed.