When you make an electric current flow through a material, the material resists, because the electrons responsible for the electric current are subjected to collisions that slow them down. This is what is called electric resistance.
Thanks to quantum physics, we are able to clearly understand the origin of this resistance and of these collisions as far as metals are concerned. The electric resistance generates heat in the material: this is called Joule’s effect and this is how toasters and light bulbs work. However, in a superconductor, the electric resistance is equal to zero and the heating phenomenon disappears, which is completely unexpected from what we had known about metals.
It was only in the middle of the 20th century that we discovered what electrons actually do in a superconductor.
