At the sea, Moses said, "The LORD will fight for you." What did the people need to do?

Answer: Be still

"The LORD will fight for you; you need only to be still" (KJV: "ye shall hold your peace").

“But Moses told the people, “Do not be afraid. Stand firm and you will see the LORD’s salvation, which He will accomplish for you today; for the Egyptians you see today, you will never see again. The LORD will fight for you; you need only to be still.””— Exodus 14:13-14 (BSB)