Backflow Prevention devices are used to protect potable water systems from contaminates should the water try to backflow into the water system.
The backflow prevention device is basically a water valve that only allows water to flow in one direction under pressure and not backwards.
It's sole purpose and job is to prevent contamination of water supplies.
Backflow prevention devices also require frequent testing and inspection especially for commercial places.
Below is a picture of a backflow prevention device.