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The power is supplied by a transformer, and when the thermostat makes contact between the 24 volt power and one or two of the other wires, a relay back at the heating/cooling unit activates the corresponding heat/fan/cool function of the unit(s).
But in addition to the 110 volt electrical supply, there may also be lights, fans, a refrigerator, or other electrical devices which use a 12 volt supply, like that of a car or truck. A 12-volt DC system powers the RV refrigerator to keep food cold while the RV or camper trailer is moving from place to place. The same goes for the lights and fans. It also keeps the on-board batteries fully charged. This system relies on the 110-volt AC electrical power supply for amperage. It is possible to pull a 12-volt power supply from the RV or camper trailer's electrical system or 110-volt campsite power supply.