A superconductor is a material that exhibits two unusual properties when cooled down to low temperatures. The graph below shows a typical resistivity trace for a superconductor. At high temperatures, the resistivity decreases slowly as the temperature gets colder. We are still waiting for a complete microscopic theory of superconductivity. Imagine a metal as a lattice of positive ions which can move as if attached by stiff springs.
Single electrons moving through the lattice constitute an electric current.
Normally the electrons repel each other and are scattered by the lattice, i.e. it resists their movement. When an electron passes by the positive ions in the lattice they are attracted and move slightly towards it. After it has passed they spring back quickly to their original positions. With some materials, when cooled to very low temperatures, the ions do not return so quickly back to their original positions and momentarily they create a positively charged local region. A second electron passing by is attracted towards this positive region and follows the first electron.