What is Fermi dirac statistics?

It would be difficult to understand the behavior of electrons without the Fermi Dirac statistics. Why in a metal, electrons can move freely to conduct the electric current and why their contribution in the same metal to the specific heat is negligible, as if their number become for an unknown reason, considerably reduced. We have here a problem of "statistical order" that can be explained only by using the Fermi Dirac statistics (the classical statical mechanics was unable to explain this phenomenon).