This is a VERY basic diagram of how a relay works. (SPST _ single pole, single throw)
you have your Main power in and out (pin 87 and pin 30).
you have your trigger power (+ and - ) (Pin 86 and Pin 85)
when you "trigger" the relay, the electromagnet forces the contact pad (the " T " in the diagram) to touch the 87 and 30, therefore completing the circuit.