The "C" stands for "Contra".
Contra in Latin means "opposite".

The B is for Body. M is for Movement. P is for Position.

In the simplest of terms it means that the opposite side of the body turns towards the moving foot. It is a position that is aquired while moving either in a forward or backward rotation. You will probably get more experience with CBMP as you continue into your dance program, but ask your teacher to demonstrate it to you. Look for this "opposition in body posture" during the movement.