Foot valves go into a reservoir or water tank and keep the input line full of water. They let the water come in but not out. This prevents the need to prime the pump or for the pump to work hard self-priming. Blumats don't like air in the lines and this prevents that. Without it, the water goes back into the reservoir each time the pump turns off and air goes in. If you install the bulkhead fitting on the side of the reservoir though and the pump lower than the water level, you won't need this. 3/4" female pipe thread.