Wi-Fi assist is iOS feature introduced in iOS 9.Use this tag for questions related with this.
Wi-Fi Assist lets you automatically pop over to cellular data if you end up on a Wi-Fi network with a poor connection on iOS devices
It was introduced on iOS 9 and one can toggle this feature on and off.
To access it make the following selections in a iOS device
Settings -> Cellular -> scroll down to the bottom of screen you can find the switch for this feature over there
