The UK mains supply is alternating current at about 230 V. It reaches an appliance through two input wires, the live wire and the neutral wire.
The live wire carries the 230 V supply to the appliance. It is the dangerous wire because its potential is far from zero compared with the ground. Its insulation is brown.
The neutral wire is close to zero volts (0 V). Its job is to complete the circuit by carrying current back to the supply. Its insulation is blue. Current only flows if both wires are connected, so an appliance needs both the live and the neutral wire to work.