a matte box is a rod supported "sun-shade" and filter holder for any camera. They could but small with an attached "flag" (or large sunshade) or fold out like a Cavision Matte Box. The purpose is to block excessive amount of sunlight that may cause Lens Flare. It is also the only way you can pratically use more than one filter (it comes with different rotating "holders") and put them on and off quickly. they look like this http://www.cavision.com/Image/Mattebox/4X5/MB4169H-2A.jpg or http://jl-site.com/Super8/Images/NizoMatt-Box.JPG.
theres my best description of a matte box. feel free to add anything i left out.