Passover, or Pesach in Hebrew, is one of three major pilgrimage festivals  that were celebrated in ancient Israel.
The holiday was originally a combination of various spring festivals and also commemorates the Israelite's Exodus from Egypt — with a special emphasis on the night when God "passed over" the houses of the Israelites during the tenth plague in which God struck down the Egyptians' first-born sons. The tradition of telling the Exodus story  has been observed by Jews ever since the Exodus  itself.
Passover's beginnings can be traced to pre-Israelite spring celebrations of the first grain harvest and the births of the first lambs of the season. Within Judaism, however, the holiday celebrates God's decision to redeem the Israelites and leading them out of slavery in Egypt toward freedom. Together with Shavuot  (the Festival of Weeks) and Sukkot  (The Festival of Booths), Pesach is one of three ancient Israelite pilgrimage festivals, during which the men of each family would journey to the Temple in Jerusalem to offer sacrifices. Since the destruction of the Second Temple in 70 CE, the focus of Pesach has shifted to the ritual meal, called the seder, that can be hosted by individual families at home or on a larger communal scale.
Because the Israelites had no time to let their bread rise as they fled Egypt, Jewish law forbids eating or even possessing any food that contains leaven. Therefore, a central aspect of Pesach preparations consists of a thorough spring cleaning to remove leavened foods  from the house and restocking with unleavened or "kosher for Passover" foods . (Many Jews prefer to "sell" their unused leaven products to a non-Jew for the duration of the holiday as opposed to throwing food away.) The spring cleaning usually consists of both a massive cleanup and the replacement of ordinary dishes used during the year with a special Pesach set.
For the first two nights of Passover, Jews living outside of Israel observe a ritual called the seder which means "order." Jews living in Israel only observe the first night. As part of the seder ritual, families read from a book called the Haggadah  which means "telling," that was compiled over hundreds of years and is often updated with modern adaptions. The Haggadah is filled with stories, songs and the blessings over various foods that are meant to symbolize different aspects of the Exodus story.
The centerpiece of the evening is the seder plate, which contains a roasted shankbone to symbolize the ancient Pesach sacrifice  in the Temple, a roasted egg to represent either the spring season or mourning (for the destruction of Jerusalem), maror  (bitter herbs) to symbolize the experience of the Hebrew slaves, charoset (which often consists of a mixture of apples, nuts, raisins, spices and wine) for the mortar the Hebrew slaves used to build the pyramids for the Egyptians, and karpas  (parsley, celery, or another green vegetable) to represent the green of spring. The seder also includes three pieces of matzah  and four cups of wine.
The number four  appears often throughout the seder. The most well known example is the Arba Kushiyot , or four questions, that are traditionally recited by the youngest person at the table. This part of the seder is meant to keep children involved and includes queries about why Jews do certain things during the evening such as reclining and dipping herbs in salt water.
Finally, there are two rituals conducted toward the end of the seder that are aimed specifically toward younger participants. There is the extra cup of wine left on the table for Elijah, which involves a child symbolically opening the door for the prophet who is rumored to visit every Jewish household for a sip of wine, and there is the afikomen — the last piece of matzah, which is usually hidden by the adult running the seder. The child who finds the afikomen gets a present, though some families give prizes to all the kids.