Spectral subtraction is a way of mixing tracks that will surgically "cut out" the frequencies of one or more tracks from another. The process will generate two or more tracks that fit together perfectly and whose frequencies never clash.
One of the main issues that arise when mixing tracks is when they share the same frequency bands: where these frequencies clash, they mask one another, in a phenomenon called Auditory Masking.
The solution to this is lowering the volume of one of the clashing track, moving it to the "background" of the mix. This is the visual equivalent of dimming one track to make the other stand out.
With spectral subtraction, the clashing frequencies are "cut out" from one of the two tracks by exactly the amount of the other track's frequency profile. Visually, it's like cutting a hole in one of the tracks to perfectly fit the other, or cutting a Dovetail Joint in woodworking.