Once a protective relay has detected a fault, signals are sent along wires and the circuit break trip coil to open the breaker and isolate the fault. If there is any problem in this transmission path (the wires and coil), then even though the protective device has detected a fault, the circuit breaker will not be opened to isolate the problem. Unfortunately any problems will only be found when the circuit is required to work.
Trip circuit supervision continually monitors the transmission path and ensures that it is functioning. If the trip circuit supervision detects any problems a warning is issued. Traditionally trip circuit supervision is carried out by monitoring the circuit resistances and sometimes voltage. Good practice would also be to monitor the circuit even when the breaker is open.