It takes place when a circuit is cooled down to near absolute zero temperatures.
Electricity works by electrons flowing through a wire, usually a metal lattice. However, at normal temperatures, the ions in the lattice will be vibrating due to thermal energy. The electrons will hit the vibrating ions and slow down. Energy will be wasted in as a result.
In a superconductive wire, the ions will not be vibrating at all or will be vibrating extremely less. This means that the electrons will experience zero resistance. However in order to maintain this temperature, wires are kept in containers containing liquid nitrogen(78K) or helium(3K).