A relay is a electromechanical device, which uses a low current coil (electromagnet) to move contacts, which in turn can control much higher currents.
On the deuce, all lights are controlled by the 3-lever switch on the dash. It has a built-in circuit breaker rated at 20 amps.
When extra lights are added, such as marker/clearance lights, this circuit breaker may run close to capacity and just stepping on the brakes or turning on the turn signals may trip it. Suddenly you have no lights at all until the circuit breaker resets, which may take a minute or so.
To prevent this circuit breaker overload, a relay can be used to handle the heavy loads and only the coil current runs through the 3-lever switch.
Circuit is shown below along with an actual relay installation (2nd image).
I have also added a relay in the turn signal circuit to reduce the chance of blowing out another flasher unit.
