Tripping in a power plant/station occurs whenever fault happens. it’s a protective measure which essentially isolates the important devices from the faulty section and thus saving it from getting destroyed (due to excessive current or voltage across it).
Tripping means the interruption in electricity supply. An electric line is tripped if it starts carrying the fault current or its get broken or due several other reasons. It is a protective measure which essentially isolates the faulty lines from the rest of the healthy sections.
Protection relays senses the fault and sends the signal to Circuit breaker and CB trips.