Hey people! I have 2

Hey people! I have 2 questions:
1) Whats mid/side EQ? Im reading and watching videos and all that but I dont seem to get it yet.. Everything goes like ”cut the low and give some highs on the sides” or sth like that and I dont seem to get the reason to do it.. Is it about compatibility? Is side my stereo compatibility and mid my mono compatibility?
2) When it comes to multiband compression.. Should all 3 bands have the same attack and release so everything moves together or does it depend on where I use it ?(its obviously that but im talking about fundamentals)..Plus, is multiband compressor the way to use dynamic EQ with a stock plug in ableton?
Thank you in advance!

James Knights Facebook fb.me/breakinscienc3

James Knights Facebook facebook.com/breakinscienc3