equilibrium - What is active mass?

I'm aware of the fact that active mass is defined as the molarity of a substance, but my textbook states that

"Active masses are dimensionless quantities but for our purposes we generally take them with dimensions of molarity, partial pressures etc."

What point is my textbook trying to get across to me? Why do active masses have no dimensions? Why have we defined this quantity in the first place, if we use it interchangeably with the term "molarity"?

Edit: Please keep in mind that I have only just graduated high school and would be very grateful if you could explain this concept to me in simple terms.