Connection whitelist is used to control certain apps accessing the internet while playing games. If you want to reduce the lags in games caused by too many apps connecting to the internet, you can turn off network connection for specific apps.
Turn on/off the Connection whitelist (for gaming)
Launch Armoury Crate > Console > Connection whitelist (under Game Genie).
Tap and enter Connection whitelist, then select to turn off the apps that do not need to connect to the internet.
In non-gaming situation, you can use Mobile Manger to further set the connection method of each app.
Swipe down from the top of the screen to access Quick Settings > tap the Settings icon > Advanced > Mobile Manager > Data usage > tap Set up button under Control app data usage. Check/Uncheck how apps connect to the internet in the Mobile data (Cellular in screenshot) and Wi-Fi lists. For example, if you only want to watch YouTube videos only on Wi-Fi, to save Mobile data, you can uncheck YouTube on the Mobile data's list.