(I do not understand relays)?
Relays are very simple. A relay is a heavy duty switch. That is all. A standard four pin relay will have terminals numbered 30, 85, 86 & 87. Terminals 30 & 87 will be the contacts of the relay, and terminals 85 & 86 will be either side of the relay energising coil.
Of 85 and 86 one will be permanent either live or earth and the other will be switched either live or earth. It doesn't matter which, but for example if one is a switched live feed then the other will be a permanent earth.
Terminal 30 is standard for battery supply, and 87 will supply the component you want to switch on. When the relay is switched on power at 30 will be available at 87 via the contacts inside the relay.
The purpose is to keep the current across the switch to a minimum, which will be a matter of milliamps.
For example, suppose you fit high power spot lamps to your car. Each lamp draws 10A and there are two lamps. That means 20A will be drawn across the switch you use to switch them on. That amount of current will cause the switch to get hot leading to failure. But if you put a relay in the circuit the switch is used to switch on the relay rather than the lamps directly. Now there are only a few milliamps crossing the switch contacts, and the current drawn by the lamps is crossing the relay contacts instead.
Hope it helps.
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