https://arstechnica.com/gadgets/2017/11/apple-clarifies-what-control-center-wi-fi-bluetooth-toggles-do-in-ios-11-2-beta/

Unbeknownst to many, the Bluetooth and Wi-Fi toggles in the Control Center in iOS 11 don't do what you expect. Rather than completely disabling those features, they only partially disable Bluetooth and Wi-Fi. To clear up any confusion, Apple appears to have added explainer pop-up messages when either Bluetooth or Wi-Fi is toggled off, as first detailed in a report by MacRumors. The messages appear in the newest iOS 11.2 beta and they explain that both features will only be temporarily disabled when turned off from their Control Center toggles.