Can anyone tell me what these formulas in electromagnetism are called?

[Ren_Hoek]

I know what $\Phi$ and B are, I think they are the magnetic flux and its density. I think $\mu$ is the permeability. But I dont know what $R_c$ and MMF are and how are these formulas deduced.

 

[Ren_Hoek]

Oh I see $R_c$ is the reluctance and MMF is magnetomotive force. I still dont know how these formulas are derived tho.