Matzah is a crackerlike, unleavened bread that Jews eat during Passover instead of regular bread. It's made with dough that never had a chance to rise. The Torah dictates that Jews may not eat leavened bread during the Passover holiday.
As the story goes, the tradition comes from a time when Jews were slaves in Egypt. When they followed God out into the desert, they only had time to bring simple dough that didn't have time to rise. Matzah is also known as "poor man's bread." So today's Jews eat matzah during Passover to commemorate the past exodus and remind themselves of faith and humility.
You can also grind up matzah to make meal, often used as a substitute for flour in Passover cooking. That's how you get the matzah for matzah ball soup.