We eat matzah on Passover night to fulfill God’s Commandment to us to eat matzah during the Passover Seder. The Egyptians fed the Children of Israel (and probably all slaves) matzah, because it takes longer to digest, and they could therefore feed them less often.
Furthermore, when God took us out of Egypt, He wished to teach us that He keeps His promises promptly, and therefore rushed us out of Egypt, without even giving us time to let our dough rise. We are therefore commanded to eat matzah on Passover night and forbidden to eat leavened bread (chometz) throughout the eight days of Passover.
The best matzah to use is the guarded matzah made by hand. Immediately upon being harvested, the grain is guarded from moisture so that it does not become leavened. The grains are guarded from moisture at all times—even after it becomes flour—until it is finally matzah.
These matzos are unlike the matzos you will find anywhere else, or anytime else. They are special Passover matzos, and they are called «shmuro matzos,» guarded matzos. They look and taste unlike any other matzos. They are not usually baked square, but round. (Though they sometimes make them square, too.)
It was the custom among many Gentiles in the ancient world to bake into their bread or matzos a sign of their religious beliefs. Many baked their bread in the shape of the idol they worshiped. One common custom was to indicate the number of idols they worshiped, by creating an equal number of corners on their matzah.
Because the Creator of the universe is eternal, Jews often made round matzos, since a circle has no beginning and no end.
For those who cannot eat wheat, shmuro matzos are also available in oat and spelt.