Pharaoh had let them go then changed his mind. He led his army with more than 600 chariots to go get the Israelites and bring them back. The people complained to Moses. Moses reassured them that the Lord would once more deliver them, and the Lord reassured Moses (14:1-18).
They know which way to go because God sends a pillar of cloud to lead them. the Lord shields them by sending the pillar behind them to block them from the pursuing army. When they get to the sea, Moses stretches out his hand, and a strong wind divides the water so they can walk across on dry land.
The people cannot see the Lord, but they can see the pillar of cloud. They can see Moses. They can see the water dividing.
William Goldingay asks: Why does the Lord use Moses, a human leader, to bring them out of Egypt? God could have done it alone but chose to use Moses. Why does God continue to use humans to accomplish significant goals? Using human beings to accomplish divine purposes introduces complication and vulnerability.
Goldingay also points out that Moses' story reminds us that leadership is not fun. People complain, question, express resentment, lose their belief that things can get better.
(Old Testament Theology, Volume One, Israel's Gospel, 425-430)