A rip current is a narrow stream of water traveling swiftly away from shore. Rip currents are formed when water piles up on the beach instead of flowing sideways away from the breaking waves as it normally does.
Eventually, so much water builds up that the pressure cuts a narrow path through the waves back out sea. The resulting current can be 30 to 100 feet wide and can move at speeds up to 5 miles per hour.
Rip currents are common around piers and jetties so it is wise to avoid swimming near these things.
The key to surviving a rip current is to swim out of it, not against it. This is done by swimming parallel to shore.
Many non-swimmers are caught in rip currents when standing in shallow water. The powerful rip currents knock them off their feet and carry them away. Since they don't know how to swim they invariably drown.