A square matrix is one where the number of rows equals the number of columns.
Computer graphics uses square matrices for many purposes. Most transformations, such as changing the point of view in a 3D world or projecting it into a 2D image, are done with square matrices.
Square matrices have properties that are not in general shared by other rectangular matrices. To get a taste of one of these properties, perform the following multiplication:
Try changing some of the elements of A (don't change elements of I) and try the product again. Only integer elements will work in this demonstration.