Today's route truck refrigeration systems in simplest terms are either cold plate or mechanical.
Cold plate refrigeration uses frozen eutectic plates inside the body compartment to absorb heat as a means of maintaining desired temperature levels. Those passive plates are "refrozen" each day during a 10 to 12 hour electrical plug in of the on board refrigerant compressor. Probably the greatest misconception or user mistake made with any variation of cold plate system is that frost or ice on the eutectic plate surface means it's cold and that's good. Not so, just the opposite is true. Frost and ice act as insulation blankets and actually prevent the plates from performing their assigned task of absorbing BTU's of heat from air inside the body space.