It's a term used to show your overall chip stack value. In the beginning, in most games, when blinds are 10 / 20 and your stack is at 1500, it costs you 30 chips per orbit. Take your 1500 stack, divide it by 30, you get 50 - you could survive 50 orbits without having to do anything. By most peoples standards if you are above 20 you are good, 10 - 20 is the moderate zone, 5-10 is danger zone, <5 is red and dead and nearing the push on any two cards. M is slightly more accurate than just knowing how many big blinds you have because it takes the ante into account during the later rounds.
there is a lot more that goes into M theory as well - "real m" takes into account raising blinds, average chip stack matters as well because turbos + 10 minute level tournys will have lower M average values than a deepstack game.