S'simple!
If you record with, say, a single SM58 mic you'll get a single channel of sound, from one perspective. That's M, or mono.
If you record with a pair of sm58s, whether crossed over or just a distance apart, you'll get 2 channels, left and right (or A and B) which are two independent mono recordings but with slight differences due to being in different positions.
Mathematically speaking, L and R added together are the same as the mono signal. But if you subtract Right from Left, you'll get the difference between the two which is known as S, or side.
So it's possible to record, or work in, M and S, which means you can boost or cut the S signal to give bigger or smaller stereo soundstage when converted back to L&R.
If you use any stereo or imaging boost software (Ozone has it) that's effectively what it's doing.