This seven or eight day festival (depending on where you live) commemorates the time when the 'Children of Israel' were led out of slavery to freedom by Moses. They had been slaves in Egypt for over 200 years and when the Pharaoh refused to release the Israelites, God visited ten plagues to the Egyptians: blood, frogs, lice, flies, livestock, boils, hail, locusts, darkness. The tenth was the plague on the firstborn.
An avenging angel would kill every firstborn man and beast but Israelite children would not be killed, and so God showed they were his chosen people. To protect themselves, the Israelites were told to mark their homes with the blood of a sacrificed lamb - the angel would then 'pass over' their homes. The Egyptians were terrified and pleaded with the Pharaoh to banish the Israelites. He summoned Moses and ordered him to leave Egypt with his people.
The celebrations during Passover, or Pesach in Hebrew, are steeped in symbolism to remember the time Jews were slaves and their escape to freedom. Special foods are eaten where families and friends gather together with singing and stories.