Stacked is the term XBMC uses within the library and file browser to enable the stacking of multipart movies, eg.
The Dark Knight - CD1.avi
The Dark Knight - CD2.avi
With stacking enabled within XBMC the filebrowser and library would treat these as a single media item and only list 'The Dark Knight.avi'
The same would also apply to
The Dark Knight - A.avi
The Dark Knight - B.avi
The Dark Knight - part1.avi
The Dark Knight - part2.avi
Then when selected XBMC would create a playlist of 2 parts, rather than having each part listed seperately and having to play each part seperately.
For some reason the XBMC team decided to apply this naming convention to fanart then to posters, and during a time period of several months various different versions worked with different formats for each but not both.
If you are using an up to date version of XBMC then it's not something you need to worry about.