With Android 6.0 Marshmallow, rather initially with the Android M developer preview which was launched last Summer at Google I/O 2015, Doze Mode was a new function that Google integrated into the system software so that users could save a little more on battery life throughout the day. The catch was that Doze Mode only worked while the phone was stationary and while the display was off, so if you were moving around with the phone in your hand but or it was in your pocket while you were walking or during other types of movement like exercise, Doze Mode would disable itself.
Things are changing with the Android N developer preview though as Google is taking Doze Mode and essentially turning it into an almost Always-On feature that will work even if the phone can sense movement. This should make it much more useful and it should affect battery life quite a bit more than before. The way that it works is that Doze Mode will now trigger anytime that a user puts the screen to sleep, or anytime the screen puts itself to sleep automatically based on the configured sleep settings whether the phone is sitting still or in movement. So long as the device isn't actually in use with the screen on, Doze Mode will work its magic.
While the new and improved Doze Mode will likely do a lot to help users with their battery life, Project Svelte is another system function under the hood that assists with better battery life by way of managing the memory more efficiently. Google states that the Android N developer preview will see them continuing to support Project Svelte and improve upon it to enhance the overall performance of the device as well as the battery. If you're someone who tends to pocket their device for a good portion of the day, you should notice significantly increased battery life with Android N, unless you already leave your phone stationary for most of the day. Anyone who tends to move around a lot will certainly benefit from this the most, but it'll be good for just about anyone who can either install the developer preview or uses a device which will end up with the official Android N software once it's released later this Summer.