All check valves are essentially used to prevent the reverse flow (or 'backflow', i.e. in the opposite direction to that intended) of fluids and gases through a pipeline system.
A double check valve is a device designed to help keep your water supply clean and untainted by implementing this very rule, but provides certain key advantages over a single check valve.
Double check valves are very common features of many modern backflow and back siphonage prevention systems, such as those found in everyday domestic/workplace equipment such as combi boilers, fire sprinklers and lawn irrigation networks.
Double check valves are less commonly used as standalone backflow prevention devices in higher-hazard applications, though, where an even more robust type of check valve system is often required in order to comply with industry codes and regulations.