Any vector may be operated on my a suitable matrix. Generally the vector is scaled and rotated. If the vectors make up the sides of a polygon then in general the polygon will be stretched or scaled in certain directions and rotated. I will illustrate this below for the unit square.
The corners of the unit square have coordinates
We can write these as vectors relative to the origin:
Suppose we operate on these vectors with the matrix
The image of
is
The image of
is
The image of
is
The image of
is
The image of the unit squareis