The Android Application Lifecycle refers to how pages and the app handles navigation and the state of the app.
The various states in an App’s lifecycle can be utilised to maintain the state of the app you would like to keep whilst also allowing the app to clear non-required pages and data from memory to optimise performance.
Here is some documentation explaining each stage in the lifecycle and when it is called in an App.
You can also view this Video.
Fragments can be thought of as screens within a screen. A Fragment can take up a portion of the user interface on the screen.
These Fragments also have their own lifecycle that should be accounted for. You can read more about that here.