The mains supply in the UK is alternating current. It reaches an appliance through two input wires, the live wire and the neutral wire, and they do different jobs.
The live wire carries the alternating potential difference from the supply to the appliance. It is at about 230 V compared with earth.
The neutral wire completes the circuit. Current flows into the appliance along the live wire and returns to the supply along the neutral wire. The neutral wire is kept at about 0 V, so it is much safer than the live wire.
