Most of the headlight relay harness use relays to send power to the headlights and they use the headlight switch to turn the relays on and off. The headlight switch sends power to the dimmer switch which then switches power between the high beam and low beam relays.
The stock system wiring has small wires to carry power from the battery to the headlight switch then to the dimmer switch and back out to the headlamps. This causes a lot of resistance and voltage loss in all that wiring and the lights aren't as bright as they should be. By using relays we can run power directly to the relays and then to the headlights and use the headlight and dimmer switches to control the relays . The relays only need a fraction of the current that the headlights use to turn on, which means that there is little drain on the current and not much loss in current for the lights.
Here is a picture of a relay and the wiring.
the original wires from the dimmer switch go to terminals 86 on the relays and terminals 85 are grounded. These little coils are all that the headlight and dimmer switch operate. When the coils close the main power wires on terminals 30 and 87 are connected and send full voltage to the headlight terminals.
Here's a diagram of a headlight relay harness showing how the power goes from the battery to the relay and then to the headlight.
