There are three pieces of matzah on the table for ritual use. We uncover them, and break the middle piece. We use the middle piece of matzah, and not the top piece, because we will make a blessing on the top piece later. Blessings are said on whole things. Now that the middle matzah is broken,  the smaller piece is replaced with the other pieces. It is the "poor man's bread" which our Passover story revolves around. The larger piece will be used as the Afikomen.

haggadah Section: Yachatz