slavery. Pharaoh would not release his Israelite slaves.
God inflicted ten plagues upon the Egyptians. The tenth plague was the killing of firstborn sons. God instructed the Israelites to mark the doorposts of their homes with the blood of a spring lamb. When the spirit of God saw these marks, He passed over these homes, hence the term "passover".
Pharaoh freed the Israelites, after which they left in such a hurry that they could not wait for bread to rise.
In commemoration, for the duration of Passover, Jews do not eat any leavened bread.