Matzah is a crisp, flat, unleavened bread, made of flour and water, which must be baked before the dough has had time to rise. It is the only type of “bread” which Jews may eat during Passover, and it must be made specifically for Passover use, under rabbinical supervision.
Eating Matzah on Passover commemorates the unleavened bread eaten by the Jews when they left Egypt in such haste that there was no time for the dough to rise. There are many mystical concepts tied to the unique relationship between Matzah and Chametz on Passover.
One interpretation equates Matzah with G-d’s commandments (Mitzvot), and Chametz with sin (Chayt). The rigorous laws of Passover, in this interpretation, represent the great care that must be taken to follow the G-dly path.