Activity Lifecycle.

An android app is divided into activities.  (iOS apps do NOT work this way.)  You can think of an activity as a "screen".  An activity has buttons and text fields and all of that to run a program or a portion of a program.  If the program goes to another screen with other buttons and stuff, then it's starting a new activity.  The current activity pauses while the new activity runs.

You can "almost" think of an activity as a method.  When one method calls another, then the first method freezes while the second methods.  However, activities aren't really methods.  Activities may contain many methods their own.  You can pass information to an activity when it begins and retrieve information from an activity when it ends...just like passing parameters retrieving a return value from a method.

The Activity Lifecycle is a sequence of methods that are run when an activity is being used.  You can't name the methods; they have already been named, but you can write the code for them.