Android Oreo allows older apps to have background limits. To be more clear, for applications which aren't specifically targeting the new Android version software with updates that revolve around it, there will now be a toggle available on the battery usage screen that will allow you to enable or disable that particular app in regards to running in the background when not in use. This is part of the changes that Google made to Android with 8.0 Oreo and is yet one more way Google is aiming to make Android that much more battery life-friendly.
The toggle is only aimed at apps which aren't targeting Oreo, so if an app has already been updated to target the new software then the toggle won't show up as an option, and there's a reason for that as those apps already have the limits enabled by default. It is also worth remembering that since the new battery optimizations in Oreo aren't meant to be implemented on older apps which aren't targeting the software, it's possible that doing so could cause those app notifications to work incorrectly.
Chances are that users who update to Oreo will have at least a few apps installed which aren't targeting the new software, and that is precisely where this new limiter will come in handy as it should help keep Google's vision for better battery life in line with how you use your device. With the toggle switched on the newly added optimizations that are part of Android Oreo will be applied to those older apps, and if for any reason you want to reverse things you can simply switch the toggle back. It's that easy, though if you're constantly fighting for battery life then you may just want to leave it on. This is no doubt a minor feature to have included and it could very well go unnoticed by more than a few users, but for those that want to squeeze every drop of battery life that they can out of their devices, this is one of the things that will assist in that endeavor, and it's a rather easy thing to do as you only have to tap the toggle, while Google's software will do the rest.