Electrical relays are devices used to turn relatively heavy loads on/off. They are triggered by small amounts of power from an external source (usually a switch operated by the user) but can pass larger amounts of power to the load.

switch (and house power) –> relay –> load

The most common relay is the solenoid that campers use to charge house batteries from the coach's alternator. They are also useful for powering loads from LVD or Charge Controller LOAD outputs.