Master Valve Wiring
A master valve is an electric valve installed at the main water supply for the irrigation system. A master valve will greatly reduce any water loss due to a leaky station valve because the leaky station valve can only leak while the master valve is providing pressure to the system. Also, if you damage the irrigation main line, a master valve will control water loss so the main can be repaired without shutting off the water supply.
An electric master valve is typically the same type of valve as you would use for your zone valves, but is installed upstream from the zone valves on the main line. It is connected to the "master" or "pump" connection in your controller -- the blue terminal in the Rachio units wall mount.