A relay is a device that allows a small amount of electrical current to control a large amount of current. A car starter motor uses a relay to solve the problem that a car has in needing a large amount of current to start the engine. A starter relay is installed in series between the battery and the starter. Some cars use a starter solenoid (as shown here) to accomplish the same purpose of allowing a small amount of current from the ignition switch to control a high current flow from the battery to the starter.