Pauli Exclusion Principle, short form:
Particles with half-integer spin (like electrons, protons, and neutrons) can't share the same set of quantum states in the same location. (It's what keeps everything from occupying the same space but instead lets them bump into each other. It's also what requires added electrons in an atom's electron shell to occupy progressively higher-energy orbitals and shells.)
Particles with integer spin (like photons) aren't subject to the principle. They can pile up in the same place and set of quantum states. (In fact, they prefer to do it. That's what light in a laser beam is: a whole bunch of photons all in the same place going the same way in lockstep.)
Conduction electrons "bump into" atoms because they have half-integer spin and the good spots are occupied by other electrons. When they gang up in pairs in a superconductor to form temporary composite "particles", the resultant electron-pair object has integer spin. So the two-electron team doesn't bump into the other electrons and can go whizzing along indefinitely (as long as its teamwork is not disturbed).
