A closed loop system is one where the ouput of the system is at least part of the input. In a refrigerator, the output of the cooling system (the cold air inside the refrigerator) is measured, and is fed back into the system to determine whether the system needs to continue cooling.
An example of a fridge that was an open loop system would be one that turns on and cools for 1 hour every 6 hours, regardless of internal temperature.
