Module files (MODs) are files that contain human-readable music data (based on pattern, page, effect and track info laid out in a spreadsheet-like manner) and sound samples. They are used to play music in MegaZeux games.
Files with the extension of .MOD are the simplest and oldest of these formats. Until version 2.48b, the only options for music in MegaZeux were ProTracker-format .MOD files (limited to 4 channels and 31 samples). The switch to BWSB for the music engine allowed several new formats, including Ultimate Soundtracker 15-instrument format .MODs and the popular S3M format. Most new formats had to be converted to an intermediary format called .GDM (short for General Digital Music).
MegaZeux switched to the ModPlug music engine at 2.80, allowing the popular formats XM and IT (as well as several others) to play natively.
MegaZeux currently plays modules with one of two possible sound engines: ModPlug, the default engine, and MikMod, an alternative module engine used when running MegaZeux on more limited platforms. For compatibility reasons, all MZX versions past 2.81h will only play the module formats both engines can play (MOD, S3M, XM, IT, GDM, STM, MTM, 669, ULT, FAR, MED, DSM, OKT and AMF).
Programs used to create modules are called trackers. Popular trackers include ModPlug Tracker (OpenMPT) and Skale. See  for a list of some others.