It clears running processes to free up ram to charge faster as well as estimate how long it will take to charge fully. I personally dont use it as it gets annoying having to swipe it away from my lock screen and then enter my pin to get into my phone. Im also pretty sure it started showing ads in that window before you can unlock the phone but its been a few updates since I have used it.
Edit:
"Charging Booster is a feature that helps your device recharge faster by killing all background tasks. When Charging Booster is active, you can:
Check what time your device will finish charging.
Access a range of other performance-boosting options.
By default, this feature activates each time you begin charging your device, and automatically kills all tasks that are running in the background. Alternatively, you can disable the feature or configure it to only activate while your device is not in use. You can also turn off automatic task killing.
If Avast Battery Saver is installed on your device, you need to manage your Charging Booster settings in Battery Saver. Otherwise, you can manage this feature in your Mobile Security settings."
I got that info from the
FAQ
