I'm a fairly n00b-level hobbyist music producer, but I can take a stab. I like thinking of the "stereo image" of a track as having 3 dimensions in physical spa

I'm a fairly n00b-level hobbyist music producer, but I can take a stab. I like thinking of the "stereo image" of a track as having 3 dimensions in physical space. If you imagine looking forward at a stereo system, left to right is the "pan" (how much of a signal is originating from the left vs. right speakers, with 50% each being dead center), volume of each track (or instrument) is how distant (or close) the sound is (you can imagine an individual instrument moving forwards toward you if louder, and vice versa), and the frequency (or pitch) is how high the track is (with lower pitch or frequency being at the floor). When a mixing engineer "mixes" a track, each track (or instrument) tends to be: 1) adjusted to an appropriate volume relative to other tracks (forward/backward), panned or spread left/right (usually more stereo "width" on the higher frequency sounds), and "equalized" to narrow and manipulate the band of frequencies coming through to the master mix from that track (it's common to cut out some of the harmonics either side of the fundamental of one instrument to leave "space" for another instrument competing for the same frequency range). So now, even with a final, mastered track, we could apply various filters to de-mix fairly easily if the mixing engineer has left a good amount of "space" in between the elements in this stereo image. If our bass is always under 150 Hz and every other element is above 200 Hz, a simple low pass will grab the bass (and likely also the kick drum, if there is one). But we could also use a "gate" to only allow a signal of a certain magnitude (volume) through to isolate that kick hit, or the inverse to exclude the kick and just get the bass sound. A band-pass could do a decent job on a guitar or vocal track, but will also grab background noise from other tracks sharing the same frequency range. More complicated techniques could be used to isolate things that have been panned left or right of center based on comparison between the left and right stereo channels of the mix. These techniques won't be as universally useful as the approach in this article, but for certain tracks or sections of tracks, can give very good results very quickly. More tracks and effects like reverb and distortion make all of this more difficult to do with simpler techniques.