In remembrance of the death of the first-born of Egypt, the Passover, and the deliverance out of Egypt the first-born of the Israelites were to be set apart. The were to be given to the service of God. There were two ways from Egypt to Canaan. God led the Israelites through the wilderness bypassing the most direct way. The Lord led them by cloud and pillar of fire.
Pharaoh thought them to be easy targets in the wilderness and set his army after them. The Israelites trapped we see the resentment begin against Moses and God. Why did you deliver us unto death? The greatness and mercy of God has been all around them, yet they cannot see it in this time where the greates faith is needed. Moses reassures them to look above for their deliverance.
When we cannot see a way through our troubles God opens a door or in this case parts a sea. The Israelites walked across the Red Sea on dry ground. The water crushed the Egyptian army as they followed behind.