Matzah is unleavened bread. The Torah commands Jews to eat matzah during Passover in Exodus 12:18: "In the first month [i.e. Nissan] from the fourteenth day of the month at evening, you shall eat unleavened bread until the twenty-first day of the month at evening." The Oral Law understands this to mean that while chametz is prohibited all eight days of Passover, matzah is only required to be eaten on the first day of the holiday.
Source: Telushkin, Joseph. Biblical Literacy: The Most Important People, Events, and Ideas of the Hebrew Bible. NY: William Morrow and Co., 1997.