The first component in any sprinkler system is the backflow preventer. Backflow preventers are required by law to protect your drinking water. There are all different kinds the one in your yard was chosen to meet your cities building code. Water is brought to your property under pressure from your cities mainline. It then enters a cross-connection with one pipe supplying water for the home and another for irrigation.
Just after this connection, on the irrigation pipe, you'll find the backflow preventer. It is important because it protects the drinking water supply by preventing water from the irrigation lines from backing up into your home causing rust, tree roots and other debris to invade your home (see the diagram below). The backflow preventer is also where you would find the systems shut off valve. This valve is used to stop the water supply to the irrigation system in case of an emergency.