Basically, is the Tello App (running on a mobile device) an optional piece of equipment or is it required for the drone to operate?
It need's a phone (or tablet), and to download the app, you can go to here. The drone is connected via wifi, and the wifi name should be fairly to recognize: 'TELLO - xxxxxx'. The app is easy to use, the left one controls the height and rotation, and the right one controls the position. There are also some warnings, including: Can't use video positioning and others.
Hope you like my answer! (Maybe I'm wrong?)