Ever since the preview release of Android M was released, there was speculation that Android M would be named Marshmallow. Well in the past couple of days, Google officially revealed that Android M will in fact be code named Marshmallow. The preview release had a code name of MNC. You would see the MNC code name within Android Studio, and many have said that MNC stood for Macadamia Nut Cookie according to this blog post, and rumored to have been the internal code name used by Google employees.
If you want to learn more about the official release of Android 6.0 Marshmallow, you should read the official Android blog here, and if you have not already done so, you should start preparing your apps so that they are ready to run on Marshmallow as it will be available on Nexus devices shortly.