MS EQ COMP can encode standard STEREO (left/right) signal to M/S (middle and side) and add EQ and Compressor separately to the middle and side, also edit the volume and pan. The output signal is decoded to STEREO (left/right).
It helps you to correct the stereo image and the sound ambience of the whole music, or add the effects to the signal of the particular instruments (for example, drums, vocal, narration, etc) on a M/S basis.
This is a particularly handy tool for tweaking the ambience of a track and more.