Wi-Fi Assist in iOS 9 detects If your mobile phone signal is stronger than your Wi-Fi signal. If it is then it will switch to using mobile data instead of Wi-Fi. This ensures you maintain a good connection at the expense of using up your mobile data. Learn how to switch off Wi-Fi assist and prevent apps that don’t need mobile data access from using mobile data.
Video – Watch how to switch off Wi-Fi Assist
From the home screen choose settings
Choose Mobile Data
Scroll down to the bottom of the menu
Turn off Wi-Fi Assist
Switch off Mobile Data for Apps that don’t need it.
The number under the apps name is how much mobile data that app has used.