From the SoundTracker web site:
SoundTracker is a music tracking tool for Unix / X11 with a design similar to the DOS program FastTracker and the Amiga legend ProTracker. The name of the program is a tribute to Karsten Obarski, who released his SoundTracker program for the Amiga in 1987 and thus unknowingly founded the basis of a huge free music movement which developed in the following years and which is still going strong.

The basic concept is simple: you have a number of sound samples, and you can arrange them on so-called tracks. A track (or channel) can not play more than one sample at the same time. Whereas the original Amiga trackers only provided four tracks (this was the hardware limit), modern trackers can theoretically mix an unlimited number of channels into one sound stream.