Google has unveiled that when it comes to Android, M stands for Marshmallow. They have also introduced Android 6.0 SDK which comes prepackaged with Android Studio or as a separate download that can be used with a different IDE.
This SDK includes the final version of the API Level 23 which introduces a number of changes over the previous version. Some of the improvements regard runtime permissions, power saving, external SD cards, the ART runtime, and others. This page enlists all behavioral changes coming in Android Marshmallow.
The hardware system images have been updated for several Nexus devices - Nexus 5, Nexus 6, Nexus 9 & Player. Developers can use these to test their applications on real devices to see if there are any issues. Now, they can also publish their apps, including those using API Level 23, on all release channels in Google Play.
We also recommend reading our previous article on Breaking Changes in Android M.