A picture frame mat is a board that is cut to have a window in it so that it borders your picture. You use them on things like pastels, watercolors, posters, and photographs. They are enhancers; basically, they add color to your artwork or keep the art from touching the glass. They come in all different colors and variations. Normally they are cut with a bevel so you see the core of the mat board. Some mats have colored core or black core, but the most common bevel is a white core. The other option is to have the mat cut with a reverse bevel. This is great when a bright white line might be distracting.
The difference between a double mat and a triple mat is that there are three mat boards stacked on top of one another. They are usually different colors. Typically there is a top mat that you see a lot of its color and then there is a middle mat and a bottom mat where you see just a small amount of its color. The amount of the middle mat and the bottom mat is called the reveal. The reveal amount will default on most mats to being a. In my opinion mat reveals should be tweaked all the time. Play with the mat reveals make them 1/8 sometimes with a really bright mat color so you can really spice up your art. Or go the other way make it a on a really big mat so you notice the bottom mat easier. You really have more options to play with reveals on triple mats.