This is the term given to cartridges which were either copies of existing official releases (often with unofficial packaging) or releases of games that never came out on cartridge, (e.g. CPC games) in professional looking packages. They often come from Poland and are sold on E-Bay.
Most likely they are existing cartridges that have been re-used. (GX4000 versions of Burnin' Rubber because these are the most common??)