Engine timing?

Now I have seen my uncle shoot a timing gun?(i think its called) on the belt of the motor when it was idling..what exactly does this measure or time?

Timing light, It flashes when your #1 cylinder fires and makes the crank look as if its standing still so you can see at what degree it is firing at.

I kinda understand what your saying,but I dont understand the crank part.

the timing light flashes every time the ignition fires on #1. On the belt pulley/crank balancer, there are numbers to give you a representation of at what particular degree the ignition is firing, with a pointer. So the light flashes, and you'll see the pointer a number or representation of a number line up and you'll know what the degree's of timing there is at idle.