I'm somewhat confused about how M/M^-1 works... in that how do they represent the transformations mentioned above? Are p_0,p_1,p_2 coordinates of a triangle in 3d space?

I'm somewhat confused about how M/M^-1 works... in that how do they represent the transformations mentioned above? Are p_0,p_1,p_2 coordinates of a triangle in 3d space?