Android M "Doze"
One of the biggest features to Android M is called "Doze" and as the name suggests, it will let the device and all apps doze off or sleep when not in use, potentially doubling battery life. Doze will recognize when a device is not in use, and put apps in a low-power state (lower than before) and stop apps or device wakes, notifications from coming through and more, and has shown to double the Nexus 9 battery life.
Doze won't stop important notifications like Gmail alerts, Facebook messages and texts from coming through, and users will even be able to manage what apps can doze, and remove certain apps from the doze list. The more apps that can take a nap, the better your battery life will be.
Apple has used something similar to this for years, which is what helps iOS have such great battery life, and similar battery saving features are coming to Android M. There are no screenshots, but know this is a new feature coming in Android M.