Well what is Mid-Side? Surprisingly, the M-S technique is one of the oldest stereo recording methods. The theoretical basis for it appears in Alan Blumlein’s seminal 1934 patent, although it wasn’t until the stereo experiments of Danish State Radio engineer Holger Lauridsen in the 1950s that the technology caught up with the theory. Mid-Side which is short for middle side is usually combination of two microphones. A figure eight or bidirectional microphone (Side channel) with a directional microphone, cardioid, hyper cardioid, super cardioid and even sometimes omni (Mid channel). The microphones sit in a vertical array, one microphone on top of the other and are usually housed within a blimp or some sort of suspension system. This technique splits the center and side information into two channels. When the signal from both microphones is decodedyou can create a multi channel signal. You can read more about how MS works here
I suggest reading this and any future posts on the matter from Mychal. Check out his full post here.