A 'tracker' is a usually-sample-based music making software. AFAIK they were originally developed by demoscene groups (concurrently to game studios that used the code privately whereas the demogroups would release binaries for others to use for music-making with few limitations) These 'sample mixing engines' were developed in order to be able to add music to their productions (demos and games), and it became it's own whole-world. Fully agree with sibling comments, module trackers can be huge fun! Near zero barriers-to-entry.. and there's still some great stuff being made in this format too!