It's when you have beat made on hardware (MPC/Triton/Minimoog/etc), and you need to record it into the computer. This is so you can do further editing, mix, master, burn to cd, create an MP3, etc...
It doesn't matter what you make the beat on hardware or software ... It's Exporting
each individual sound used on it's own track (file) for the mixing process so everything
can be mixed on it's own channel. For example your beat consists of a kick, snare, hat,
& a synth. When you export your beat to listen to you usually do it in 1 mp3 or wav with
all the sounds together. When you track out you export each sound in it's own file
so you'd have 1 wav just the kick playing, another wav with just the snare, and so forth.
Then who ever is mixing the track would load up each wav (kick, snare, hat, synth) into
the sequencer each on it's own channel to mix.