Mid-side (MS) processing is a special method of processing stereo signals. The purpose of MS processing is to decompose a stereo signal into different parts.
On its own, MS processing does not actually change the signal. However, it is meant to be used with other processing like equalization or compression.
When using MS processing, a conventional stereo signal with a left channel and right channel is separated into a mid channel and a sides channel. This way it is possible to equalize or compress the sounds in the center of a mix independently of the sounds on the sides.