A circuit breaker is the more convenient alternative to a fuse. Like a fuse, it breaks a circuit if the current rises above a set level. Unlike a fuse, it does not need to be replaced. It can be reset by flicking a switch, avoiding the need for unscrewing a plug, and maybe finding you have no spare fuses.
When a large current flows in the coil around the electromagnet, the electromagnet is magnetised and attracts the right hand contact, forcing the bottom end over to the other side of the spring and breaking the circuit at the contacts. The circuit breaker is reset by pulling the spring, allowing the bottom of the contact to move back across the spring.