I am not going to borne you with too much talk but to practically tell you how automotive relays work.
First of all know what a relay is.
A relay is an electromagnetic switch that makes an electromagnet when current pass through the coil. It either close or open a circuit.
So, we have a normally close NC and a normally open NO relays.
The main reason why we need relays in our circuits is to amplify small current produced by sensors a to control other devices that requires larger current. Using a small amount of current to control devices that need large amount of current.
A relay is an amplifier or a switch for the reasons measured above.
Above is how a relay makes contact.
WORKING PRINCIPLE
Position 1 is the coil that needs to be energize with a battery negative and positive before position 2 can make a contact. Position 1 is usually terminal 85 and 86 (battery negative and positive). Position 2 is usually terminal 30 and 87 (30 is battery positive while 87 goes to the particular device that you want to control with the relay). This is for a 4pin realy.
From the diagram above, it is normally open when it hasn't make a contact and it is normally closed when it has make a contact.
Please add on the comment what you know about a relay . Thanks.