What does it mean to "Hibernate" apps?

More specifically, if I hibernate an app, does it "wake up" as soon as I begin to use that app? Or what? I need more information about "Hibernation" of apps. What happens, etc.

Basically, when running the Booster process, CCleaner sends the selected apps a hibernate request. The app can then do one of three things: shut down immediately, shut down at a later time when it's convenient, or disregard the request.

In any event, it's basically the same as if you were to close the app yourself via the App Overview in the Android OS, and so it won't cause any problems with your apps (though it may take them a few moments to start back up afterwards since they have to start 'from the beginning').

This will typically stop any running tasks the app might be doing, if it accepts the hibernate request, but nothin should stop it from opening back up again in the future. We have a bit more information in our Help Center here: https://support.piriform.com/hc/en-us/sections/115000990092-Booster about the feature.