It sets the page in a way that inputs and other elements behave for optimal mobile use. Like when you tap onto an dropdown element and an iOS type of mobile menu shows you options.
The engine has evolved over the years to a point in which this happens pretty much in all the UI elements without having to activate this setting.
This is also used mainly when an app is designed as a single page app (designed with the UI of a native app) so that it can be wrapped to be published in the app stores.