This series of threads is for the complete dummy. There are too many people out there asking about fog light wiring. And some people have no clue. And others who think they are the sh!t got it completely wrong.
The most basic electrical circuit you can have is with a wire and resistor.
A resistor impedes the flow of electricity. If there was no resistor in our circuit, the current would a flow freely. We would create a condition called a short. The free flowing current will cause the wire to heat up and melt.
If you take a open box wrench and laid it on top of the POS and NEG terminals of a car battery... Watch out. The wrench is going to melt.
A relay is a switch. But unlike a "conventional" switch that you have to flick with your finger to close the circuit, a relay is an "electronic" switch. A relay contains a switch and a resistor. When electricity passes through the resistor, the switch closes to complete the circuit.
In the example, when electricity passes through the resistor in the relay, the switch in the relay closes to complete the circuit and light up the bulb.