Android M stands for android marshmallow.
Some of features are:
Display and app privacy
The app menu, for example, is now a single vertically scrolling screen. The 'Do Not Disturb' feature for notification muting has been improved too. One of the most important changes, however, is the new method for managing app permissions.
Users are getting more and more privacy-conscious, with an increasing awareness and mistrust of how much of their personal information their apps have access to. Google has acknowledged this, and now rather than simply granting a swathe of app permissions during the initial download, users can choose to allow or deny access to specific functions.
There's also split-screen windowing, as seen on devices like the Galaxy Note 4, and a boost to standby power in the form of Doze. Since this mainly deals with polling updates while the device is in sleep mode, this is largely going to be more useful for tablets than it is for phones, but it's still a welcome addition.
There are several more features too.