As tradition has it, after decades of slavery to the pharaohs in Egypt, God sent Moses to tell the pharaoh to release them. The pharaoh resisted, and God set 10 plagues upon Egypt. One was the sacrifice of all first born sons, but Jewish households were "passed over." Finally, the Jewish people were released and undertook an exodus to freedom.
Today, the first night is celebrated with a seder in which traditional foods are consumed and the story of Passover is read.