Passover commemorates the freedom of the Jewish people from bondage in Egypt. The term Passover refers to the Angel of Death passing over the Jewish homes marked with lamb's blood and killing only the first born sons of the Egyptians, including the Pharoah's son. The death of his son was essentially the last straw in a series of misfortunes and the Pharoah let the Jewish people go free.
People prepare for Passover by cleaning their homes and removing all leavened foods.
Leavened foods include things like yeast bread and cakes. You can practice removing the leaven with this online game . They eat special foods and read a Haggadah. For fun, check out Uncle Eli's Haggadah . The above graphic is a representation of a vegetarian Seder plate.