One of the brand new features in Android 5.0 Lollipop, is the battery saver. It's something we've seen for quite a while from other OEMs and even some apps. Although this year saw the beginning of the "Extreme Power Saver" type features. Google finally added a Battery Saver option on Lollipop this year, as part of Project Volta. It's a nice battery saver, it does turn off some auto-sync capability, as you'd expect, but it does save you some juice so that you can get home and not have a dead phone. I've hardly ever used it on the Nexus 6 for the two weeks I've had the device. But if you use your device heavily, it's nice to have this feature.
So how do you enable it? Well you can pull down the notifications and tap on the battery (or go the long way and tap on settings then battery), and from the battery screen, tap on the overflow menu on the right side. The first option will be "Battery Saver". The next screen will tell you what the battery saver does, it also automatically turns off when it gets plugged in. Additionally, You can choose to turn it on 5 or 15% left. I typically leave it on Never, because it'll prompt you when you hit 15% anyways and you can turn it on straight from your notifications there.
It's a really nice feature to have. Like I said, it's important for when you have a long day as it'll help you keep your phone alive all day. Still wish there was a way to put the battery percentage in the notification bar without rooting though. Hopefully that comes in Android 5.1.