Sometimes a movie is filmed in a full frame version and then later mattes are added to the top and bottom to create a widescreeen aspect ratio. Usually when fullscreen versions of these films are released on home media (DVD, Bluray, etc.) the mattes are removed to restore the fullscreen picture. Thus in these types of fullscreen versions you aren't actually losing any picture information as you would with Pan and Scan fullscreen versions. In fact, you are seeing more of the picture than would be seen in a widescreen release. For a more detailed definition of Open Matte see the Wikipedia page here.
