Normally metals have some electrical resistance, even realy good conductors. Resistance means whenever electricity flows through, they heat up and some electrical energy wasted as heat.
If you make a metal cold enough, the resistance completely disappears and the metal becomes a superconductor. With no resistance, electrical energy turned to heat and none is wasted.
Means you can start a current flowing through superconducting circuit, take out battery and current would continue forever.
Using superconducting wires you can make power cables that transmit electricity with no loss of power (loss-free power transmission), really strong…