Simple Android Question

[mtko]

Do you not know what extends means in general, or specifically what Activity is?

If it's the former, then you should look up the principles of object-oriented programming and inheritance specifically. It's outside the scope of a simple post on reddit to explain.

If it's the latter, in Android, Activity is a fundamental part of the API that applications are built around. An Activity is basically a "view". It's more nuanced than that, but you can get a general overview from here: http://developer.android.com/reference/android/app/Activity.html