"Limited to the number they can sell."
Films and video games are commonly being released in two versions: the Vanilla Edition, and a better, souped-up edition with exclusive Bonus Material. With movies, you can expect deleted scenes, filmmaker interviews, never-before-seen footage, commentary tracks and so forth. Video games tend to offer a shiny metal case, developer artwork, bonus characters or the Strategy Guide. Concept art, a "making-of" featurette, background story information, character biographies, alternate endings and trinkets are common with both media.
Of course, you usually have to pay US$5–$20.
Worth it? Depends on the individual work, as well as who you ask. In any case, this is a good marketing strategy, because it feeds into many people's compulsion to own the "best" version. It's particularly clever if you can get people to buy the "ordinary" version, discover how cool it is, and then go shell out again for the Limited Special Collectors' Ultimate Edition.