You may remember the post about Framing? Sometimes, as with work on paper, including a mat (or mount) in the frame can really highlight the work. Matting can also detract, for example, if the white of the mat and the paper are not a good match, one Consider the option of framing without a mat. Your framer will help you decide.
If you do include a mat, it may help protect the work from any moisture that gets in the frame, and keep any of the pastels, etc. used in the work off the inside of the glass. They can also give the image room to 'breathe', some relatively blank space between the art and the frame. Most mats are paper-based, solid colour. A few are metallic, fabric covered, flocked, or textured, and/or patterned papers.
Usually, mats are single or double. A double mat allows for the top mat, the one you see most of, to be one colour, and the bottom mat, of which you generally see just a thin accent, to be another colour. It depends on the colours in the artwork or print being framed, but often the top mat is a softer or neutral colour, and the bottom a darker, or brighter, colour. These should always pick up on colours in the artwork. Try picking up on a secondary, not a main, colour of the work - if the art is mostly blue, but has some red in it, using the red as your mat will make that colour pop in the art. The photo in this post is an example of a student work of mine, which is unsurprisingly colourful, matted with a warm white top mat, deep pinky red bottom mat, and blue painted frame. The second of two prints in the pair, is even more brightly framed, picking up on a bold pink, and bright mid green.