The name of the holiday refers to the last of the 10 plagues against Egypt, when God smote every first-born Egyptian male, but "passed over" the homes of the Israelites.
On the holiday, Jews traditionally gather with their families for a meal called a Seder, during which participants read the Haggadah, which tells the story of Passover.
For the duration of the holiday, Jews are forbidden from owning or eatting any leavened products. This is a reminder of the story of the exodus from Egypt, when the Israelites left so quickly that they did not have time to let the dough rise. Instead of bread, Jews today eat matzah.