Fermi level is used to describe the top of the collection of electron levels at absolute zero temperature. This concept is derived from Fermi-Dirac statistics.
Electrons are fermions that cannot exist in identical energy states. At absolute zero, these electrons pack into the lowest energy states available and build up a “Fermi sea” of electron energy states. The Fermi level is the surface of this sea at the point of absolute zero, and none of the electrons have enough energy to rise above this surface. Fermi energy is an important concept for understanding of thermal and electrical properties of solids.