I'm glad you asked, some people are to shy to ask a "simple" question- but "no question is a stupid question" I always say:
Dulcoat (or flat-coat, matt-coat amoung other names) is a final coat many modelers apply to their model for a couple of reasons.
-seals the decals and other applications of weathering (chalk, pencil, etc.)
-gives the model an overall dull sheen, which may give a scale efffect, consitant look, and/ or replicate the actual matt look of many military vehicles.
-It is usually applied by airbrush or spray can. I do airbrush as it gives the most percise control of where it goes and thin coats
Most hobby shops will cary one type of dull/matt/flat coat or another. I prefer acrylic-based ones from Model Master. You can also buy it in spray cans at your local Hardware store. Krylon is a common brand.
Various sheens of semi-gloss or "satin" are also availalbe.
Alternatively, you may desire a gloss finish on your model. A coat of gloss is a good idea to lay down before decaling as well to reduce the chances of the decals "silvering". I typically use Future Floor Wax for this. You can search the forums for many discussions on Future.
"I love modeling- it keeps me in the cool, dark, and damp basement where I belong"
Current Projects:
1/48th Hasegawa F-14D- 25%
1/48th Tamiya Spitfire- 25%