Pesach celebrates the Biblical exodus from Egypt, where the Jewish people were slaves. The seven- or eight-day holiday (depending on whether one is celebrating in Israel or elsewhere) begins with a festive meal called a seder ("order," because the evening follows a specific ritual pattern) on the first and/or second night. The accompanying guide is called a Haggadah, or "telling."
Specific foods are eaten during the seder and throughout Pesach to commemorate various aspects of the Exodus. Because dough did not have time to rise while the Jews fled Egypt, matzah (an unleavened cracker made of flour and water, baked for no more than 18 minutes) is eaten instead for the duration of the holiday, while leavened items made of wheat, rye, barley, oats or spelt (bread, pasta, etc., collectively known as chametz) are avoided. Some choose to avoid foods made with other grains like rice or corn as well.
Other symbolic foods include a green vegetable (signifying the coming of spring) dipped in salt water (like the slaves' tears), a bitter herb called maror (to remember the bitterness of slavery), and charoset, a pasty mixture of fruit, nuts, and wine that resembles mortar used by the slaves to make bricks. Z'roa, a lamb shank bone, represents the animal sacrifice whose blood was painted on Jewish doorposts so the Angel of Death would know to "pass over" those homes during the tenth plague on Egypt (Death of the Firstborn) and later the animal sacrifice brought to the Holy Temple in Jerusalem.