I've had my iPhone 6 Plus for about 1.5 years now, initially the battery life was amazing. More recently I've noticed that it rapidly drops off, this is in part due to the age of the device but it's also in part due to several factors that I will cover in a series of posts about preserving iOS 9 battery life. I expect these tips will remain valid even with subsequent iOS versions.
Background App Refresh was a feature introduced in iOS 7 and for the best summary of what it does I refer to an Apple support article:
...Apps can continue to run for a short period of time and are then set to a suspended state so they are not actively in use, open, or taking up system resources. They will instantly launch when you return to them. Certain tasks or services can continue to run in the background. To lessen the effect on battery life, normal app background refreshing is scheduled for efficient times, such as when your device is connected to Wi-Fi, plugged into a power source, or being actively used. When Background App Refresh is on, apps that take advantage of this feature can refresh themselves in the background. For example, an app can check if new content is available and download the updates, or retrieve the updated content in the background when it receives a push notification, so the new content is ready for viewing when you launch the app. Apps can also schedule background refreshing based on your location. If you force an app to quit by dragging it up from the multitasking display, it won't be able to do its background activities, such as tracking location or responding to VoIP calls, until you relaunch the app. iOS learns patterns based on your use of the device and tries to predict when an app should be updated in the background. It also learns when the device is typically inactive, such as during the night, to reduce update frequency when the device is not in use.
You will read many articles on the internet that tell you disable all Background App Refreshing to save battery life and others that will tell you not to bother at all as it makes no difference.
Apple Resource's advice is to go through the list and make a conscious decision as to how much you use each application and how much you'd value it updating whilst in the background. It's perfectly OK to leave apps that you value and use all the time as active due to the intelligent way Background App Refresh works. What you don't want is every single app using it if you rarely use them. Although it was first introduced in iOS 7, apps taking advantage of Background App Refresh will have percolated through over time, which is why you may have a shock when you open up the Background App Refresh found at:
Settings -> General -> Background App Refresh