Most Apple devices do not need calibration. Their crystal oscillator pitch reference is adjusted well enough at the factory so that the pitch reference is close enough. But if you want to remove even that tiny difference, you can use Internet Calibration.
When you tap on the Internet Calibration button, a new screen is presented that describes what will happen next and what you need to do to use Internet Calibration. Basically, you have to leave your device running the calibration uninterrupted for six hours. Therefore you would probably only do this overnight when your device is not needed for anything else. The device will also have to be on the charger because it may run the battery down in six hours of running.
If you decide that you can do this, then tap on the button that says "Begin 6 hour Calibration". Wait for a few seconds to make sure some error message does not appear right away, then leave your device on and running the calibration until the next day.
The next day, you should have an alert asking you if you want to accept the result of the calibration. The alert message will tell you how much of a correction is being made. If you accept that calibration, it will be stored on your device and used thereafter to make the pitch measurements as accurate as they can be.
It is also necessary to have Internet access at your device during the calibration. The Internet access is used once briefly at the beginning of the six hour period to acquire a time reference, and again briefly at the end of the six hour period to acquire another time reference. During those six hours, the software will count audio samples to calibrate for the actual audio sample rate, which is the pitch reference used in Montal Tuner.