The service valve on your air conditioner is a group of ports where the HVAC technician can hook into your system to figure out the problems with it. The service valves are generally located on the refrigeration lines near the compressor on the outdoor unit of your central system. They are simply openings where professionals can attach their equipment to the air conditioner without interfering with its operation.
The HVAC technician can find out a lot of information about your air conditioner through these service valves. Located on the high and low pressure sides of the unit, the technician can use these access points to safely remove refrigerant in the event of overcharging. They can also force new refrigerant into the lines for replacement or add some if a leak has caused some of the refrigerant to escape. They are also able to help the technician measure the pressure of the refrigerant gas in the system. Much like a valve on a tire, they are airtight unless they are attached to a specific type of connection that opens the orifice.
The service technician carries a bundle of hoses attached to a pair of gauges to test and repair your air conditioner at the service valves. The ends of the hoses attach to the gauge mechanism on one end and lock into the service valves or ports on the other end. Using a hand release, the hoses open up the service valves and allow the refrigerant pressure to reach the gauges either on the low or high pressure connection. The gauges read the refrigerant levels and let the technician know whether the charge is sufficient or if recharging with more refrigerant is needed. A center hose can connect to a service port used for removing or adding refrigerant to the system. The refrigerant can be sucked out safely to a storage tank or new refrigerant can be pumped in. Also, nitrogen may be used to blow out the system entirely and clean it out through this same service valve.