A plumbing check valve is a device which allows liquid to flow through in only one direction.  One need not know anything about plumbing to understand the importance attached to fluids in your home only flowing in one direction.

Fluids (mainly water) flows, normally, from the city’s potable water supply to your home by means of water pressure.  All is well until that water pressure decreases, at which point a phenomena known as backflow occurs, meaning the pressure reverses and contaminated water flows from your home back to the city’s water supply.

A plumbing check valves prevents that from happening, and that is a very good thing indeed.