The answer is a circuit breaker, which literally "breaks the circuit". The reason these are important is that if you have a device that can fry if too much current hits it, the circuit breaker prevents that from happening. For example, if lightning hits the power lines, it will send massive amounts of electricity through, which can fry computers, phones, microwaves, and even electric ovens. The circuit breaker will trip, stopping your electronics from being destroyed.
