Solenoids open and close automatic irrigation valves. They operate via a 24V electrical connection to an irrigation controller. When electricity flows from the controller to the solenoid, a small steel rod inside of the solenoid lifts off a bleed hole, allowing the valve diaphragm to open and water to flow through the valve. These replacement solenoids are for 24V irrigation valves. They have a stainless steel rod, copper core, insulated lead wires, and a waterproof resin casing which protects the internal components. Interchangeability of 24V solenoids depends on the footprint. See compatibility chart in the images.
While it is relatively rare for a solenoid to fail, replacement is very straightforward. Simply turn the water mainline off, remove the waterproof nuts from the wires of the existing solenoid, unthread the solenoid from the valve, thread the new solenoid into the valve, connect the solenoid wires with new waterproof wire nuts, and turn the water mainline back on. For outdoor irrigation use with cold water only.