You can think of the Fermi level as the center of gravity of the amount of free charge particles. i.e.
If the Fermi level is in the middle, then there is a balance between the number of free charge particles in the valency band (holes) and the number of free charge particles in the conduction band (electrons).
When it tends to be more shifted towards the valency band then there are more free charge particles in the valency band (holes) than in the conduction band (electrons) and vice versa.