M is an internal bit(coil) you can energize(set/reset) these bits and use their contacts. There are also alot of dedicated M coils in the plc's such as M8000 (plc run contact). Check the manual for the plc you have.
You really need to go to the Mitsubishi website and download the Programming manual for the type of PLC you are using. There are many different kinds of internal latches and special functions. M is just the basic one. Regards Badger.