Enough said about that but I still haven't discovered what the term mat means.
Creating a MAT is achieved by placing your image in the queue and a right click on it. That will give a menu option to create a mat
It actually says Photo Mat create uneven borders.
The beauty of using mats is that you can create any thickness, any colr and apply to all sides or one or two sides. You can make a thick border on the bottom and add floating text on it. Many things to play with.
Not the least of which is multiple mats. add another mat and another and another, various colors and thicknesses.
You have to try and use it a few times.
Fred