CLEO is a library of functions that allows developers to create custom scripts for GTA SA. It provides an interface between the game's code and the modder's scripts, enabling the creation of complex modifications. FLA files, on the other hand, are archives that contain a collection of files used by the game, such as textures, models, and audio files. By modifying or replacing these files, modders can significantly enhance the game's graphics, sound effects, and overall performance.