A fuse is an electrical safety device. It normally consists of a small cylinder containing wire. If current in excess of the fuse's rating passes through the fuse, it 'blows' shutting off power to the circuit. This prevents damage or fire caused by excessive current passing through wires or equipment such as might be caused by a short circuit.
Fuses are one shot devices - they have to be replaced. Early electrical circuits used fuse wire with a similar function, modern electrical circuits often use circuit breakers instead which can be reset without requiring replacement.