Android 5.1 vs Android M - Fingerprint Support
Android M will finally have native fingerprint support for devices with a fingerprint scanner. This means we won't have to rely on Samsung's options for the Galaxy S6 fingerprint scanner, and more manufacturers will likely add this as a security option.
With Android M users can unlock the device, make app purchases, sign into websites or accounts, use Android Pay and more all with their fingerprint. So long as the device has a fingerprint scanner. There's no screenshot for this on Android 5.1, and M doesn't have it yet as there's no fingerprint scanner on current Nexus devices.
This should bring improved security, convenience, and better Android Pay usage to Android devices.