iOS App: what is it for?

iOS is not an app, but the operating system of your device, it is managing everything your device is able to do

All basic Apple apps are good for something, like the Clock for everything regarding times, the calendar for calendar functions, the contacts for everything around contacts, etc…

best you read the manual iPhone User Guide - Apple Support (NZ)

In addition to the excellent answer given above, if you tell us what you want to do with your phone, perhaps people can make suggestions about their favorite apps for that particular purpose.