Check valves are mechanical valves that permit water to flow in only one direction, preventing process flow from reversing.  They are classified as one-way directional valves.  Fluid flow in the desired direction opens the valve, while backflow forces the valve closed.

These install near your skimmer or filter, so if the pump is turned off, it will not back-siphon and dry up.

Also used to help with priming an external pump. Used on the intake to keep water in the pipe.