As metals are cooled, their resistance decreases.
It was discovered as long ago as 1911 that when mercury was cooled using liquid helium to 4.1 K (4.1 degree above zero), its resistance suddenly falls to zero.
This phenomenon was named superconductivity. Other metals, such as lead at 7.2 k, aslo become superconductor.
When a current flows in a superconductor it can continue in that superconductor without the need for any potential difference and without disscipating any energy.
This means that large currents can flow without the unwanted heating effect that would occur in a normal metallic or semiconducting conductor.
Initially superconductivity was only of scientific interest and had little practical use, as the eliquid helium which was required to cool the superconductors is very expensive to produce.
In 1986 it was discovered that particulat ceramics became supercunducting at much higher temperature, above 77K, the boiling point of liquid nitrogen.
This means that liquid nitrogen, which readily available, could be used to cool the superconductors and the expensive liquid helium was no longer needed. Continuously superconductor technology became a feasible proposition.