It's in two parts. A plastic-covered sensor clips round the live wire (round the plastic insulation so it's quite safe). The sensor is attached to a battery-powered transmitter, which is stuck on the wall next to the sensor. It must clip round either the live or neutral wire. If you clip it round both it will read zero.
The display picks up the signal from the transmitter, and displays the amps. If you tell it the voltage (eg 240 volts) it can show the power being used, and the power consumption over time. If you tell it the cost per unit it will even show the cost of the power used. The batteries seem to last years rather than months