Generally speaking it is a kind of entry point into your application. This class manages the state of your application. It also contains the main window of your application, that is the
. In this class configures all the basic needs of your application, whether it is Push Notifications or Voice Calls, and so on. I would say this is the Core of your application.
You can tell from his inheritance.
On the first:
An abstract interface for responding to and handling events.
A set of methods that are called by the singleton UIApplication object in response to important events in the lifetime of your app.
That is how I feel there is an important(core) things in your app.
But at the initial stage of the study it you almost do not need.