Electricity flows from a battery in one direction only, and some components work only if the flow through them is in the correct direction. This acceptance of a one-way flow is called polarity. On most cars the negative () battery terminal is earthed and the positive (+) one feeds the electrical system.
The fuse box is often located in a cluster of components, as illustrated here. The box is shown with the cover off.
Apart from the main charging, starting and ignition circuits, there are other circuits that power lights, electric motors, the sensors and gauges of electrical instruments, heating elements, magnetically operated locks, the radio and so on. All Circuits are opened and closed either by switches or by relays - remote switches operated by electromagnets.
The sudden surge of high current in a short circuit makes the fuse wire melt, or blow, breaking the circuit. When this happens, see if there is a short circuit or a disconnection, then install a new fuse of the correct amperage rating (See Checking and replacing fuses).