Whenever a window is cut in a mat board, we always cut the window at a 45 degree angle. By default, the bevel is exposed so you can see the core of the mat along the perimeter of the window. On a mat with a white core, this bevel is a bright white line.
A reverse bevel is simply a mat that has been cut in the opposite way, so that the bevel is facing the art and not visible in your design. On some artwork, the bevel can be a distraction, especially a white bevel on dark imagery, so choosing a reverse bevel can be a thoughtful choice.