A fuse is a thin, tiny conductor designed to melt and burnt into two pieces for the purpose of breaking a circuit in the event of excessive current. Under normal operation, the fuse is no different than other wires. But when the current passing through the circuit is getting too high, the fuse will be getting hot as well, and eventually hot enough to melt and burnt into two pieces so the circuit is cut off. Because the fuse is sealed into a glass tube, even if it burns it won't cause a fire since no oxygen is inside the tube. Once problem of the circuit is fixed, replace the broken fuse and the circuit will be good to go again. Remember, DO NOT use a regular wire to replace a fuse, otherwise whenever current gets too high and there's no fuse protecting the circuit, the entire circuit will get overheat eventually and likely cause a fire.