A lib or library is a file for The Games Factory, Multimedia Fusion, The Games Factory 2, or Multimedia Fusion 2, consisting of several click Objects, generally for the purpose of using their graphic data (generally sprites and some backgrounds.)
In earlier eras of fangaming, libraries were used extensively, and on MFGG were often submitted as comical parody games whose sprites could be used if required. However, the comical nature began to supercede the practical nature over time, leading to questionable content such as Super Saiya-jin 4 Mario, and absolutely worthless content such as Pregnant Amy Rose.
The use of libs died out rapidly after the site began encouraging sprite sheets for compatibility with the increasingly popular GameMaker, because libs of later programs (MMF2 etc.) cannot be used in earlier programs (TGF etc.), and because sprite sheets are far easier to use for developers anyway, compared to one enormous library that must be searched through.
Libs also exist in Game Maker. The default number of libraries for Game Maker is 7, but users can download more and put them in Game Makers library folder, and use them in Game Maker's interface.