It gets a .usf file, and through some trickery and maths, turns it into a piece of music you can hear! It works similarly to trackers, in that songs are made up of patterns, and each pattern has a certain amount of tracks in it. These tracks contain all the instructions that tell it what sounds to play at what time, as well as how to play them. Each pattern can be of any length, and have as many or as little tracks as you like; it'll fill in the blanks automatically. (with literal blanks, the actual composition is up to you)
The whole thing runs from the command line. The package includes a bunch of utilities that basically transform your terminal into a "music production suite". These include recording and playback utilities, effects such as echo, timestretch, loudness and a basic low-pass filter, and the ability to truncate, join and mix samples. Armed with a text editor, you are unstoppable.
Perhaps the biggest omission compared to trackers is that it can't play samples chromatically. This was never intended from the beginning, and will never be added because the maths is way over my head, and that kind of stuff is totally outside the scope of this project.