When you turn on zone 1 from the clock/timer, it opens up a valve that directs water to about 6 heads, and so on for however many zones you have.

Valves are almost always in valve boxes in the ground somewhere near the grass, or in the grass. The boxes should be flush with the ground and usually have green plastic lids, usually 12"x 18" rectangles.

Lift the lid off and there will be anywhere from 1-3 valves inside the box.

This is what typical valves look like:



On some valves, there is an adjustable screw on top that regulates the amount of water that flows through the valve. The first thing I would check is whether that flow control is all the way open allowing max water through. BUT, if you don't even know what a valve is or where it is, it's unlikely that you would have closed that without knowing what you were doing. Maybe some random kids did, but it's unlikely. And a lot of valves don't even have that on there, they just always allow max flow, you can't regulate it.