The name Pesach is derived from the Hebrew word pasach, which means "passed over," the source of the English name for the holiday. It recalls the miraculous tenth plague when all the Egyptian firstborn were killed but the Israelites were spared.
The story of Passover originates in the Bible as the telling of the Exodus from Egypt. The Torah recounts how the Children of Israel were enslaved in Egypt by a Pharoah who feared them. After many generations of oppression, God speaks to Moses and instructs him to go to Pharoah and let God's people go free. Pharoah refuses and Moses brings down a series of 10 plagues on Egypt.
The last plague was the Slaying of the Firstborn; God went through Egypt and killed each firstborn, but passed over the houses of the Israelites leaving their children unharmed. This plague was so terrible that Pharoah relented and let the Israelites leave.
Pharoah then regretted his decision and chased the Children of Israel until they were trapped at the Sea of Reeds. God instructed Moses to stretch his staff over the Sea of Reeds and the waters parted, allowing the Children of Israel to walk through on dry land. The waters then closed, drowning Pharoah and his soldiers as they pursued the Israelites.
The Torah commands an observance of seven days of Passover.
(text from: https://reformjudaism.org)