Hi:
A relay is basically a heavy duty switch designed to handle more current than, say, your headlight switch. If I understand the wiring correctly, on many cars relatively minimum guage wire runs to the headlight switch, then to the headlights. This restricts the amount of current that gets to the lights. With a relay, heavy guage wire runs to the relay, then to the headlights. The light guage wire that used to run from the switch to the headlights now runs to the relay, and turns it on as a low current device, and the relay turns on the headlights as a high current device. This allows the headlights to draw the maximum amount of power without cooking the wiring or headlight switch. There is a great discussion of relays and diagrams for wiring at http://lighting.mbz.org/tech/how_to/relays/

Hope this helps

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JCE
87 300E, 65k miles
Smoke Silver