(or Pesach) is a Jewish holiday celebrated in the spring which commemorates the ancient Israelites' Exodus from Egypt. During the week-long holiday, only unleavened bread (matzo) is eaten and all chametz (bread or other food made with leavened grain) must be avoided.
Chemically speaking, all breads, cakes, etc. in which leavening agents are used are foams that go through an autolyse step (either being allowed to rise before baking, or puffing up in the oven while baking) and solidify with the application of heat and hold their structure. Made without using yeast, baking soda, or other substances that would cause a foaming reaction and cause the dough to puff up, matzo dough is mixed quickly and rolled out without being given time to rise (the autolyse step commonly associated with making bread is skipped).
The thin, crispy matzo cracker, as well as matzo meal (ground matzo), have loads of applications during Passover and the rest of the year too. Matzo Toffee (also sometimes called Matzo Brittle or Matzo Brickle) is an amazing combination of toffee, chocolate, and nuts that's just so amazing (yet simple) that you'll likely end up making it year-round.