Hybridization happens when atomic orbitals mix to form new atomic orbitals. The new orbitals have the same total electron capacity as the old ones. The properties and energies of the new, hybridized orbitals are an 'average' of the original unhybridized orbitals.
The concept of hybridization was introduced because it was the best explanation for the fact that all of the C - H bonds in molecules like methane were identical.