These typical valve boxes are used with the Irritrol RainDial RD600, RD900, and RD1200 models.   Valve boxes contain the valves and each valve has a solenoid that controls the water flow.   The sprinkler timer, like the RD600, activates the solenoid by applying 24VAC to the wires connected to it.  The sprinkler timer is simply a 24VAC switch with some fancy scheduling capability.

Valve boxes

Valves and wiring inside the valve box that is controlled by the Irritrol RainDial.

The wires go to the solenoids.  The other end of the wires go to the timer.

Typical Valve and Solenoid

A typical solenoid; it screws into the top of the valve.  The timer connects 24VAC to the two solenoid wires to make the valve turn on.  The solenoid is an electrically controlled water switch. The RD-600 series of sprinkler timers are what turns on and off this “switch” according to the timer’s schedule that you programmed.

Valve with solenoid shown circled in red

valve and solenoid with backflow prevention built in.

Backflow Prevention Valves for a sprinkler system

These prevent water in your irrigation system from flowing into your house in the rare event that your house water pressure dropped, typically from a broken water main, etc.

You can see information on these on this page: link