Android action.MAIN and category.LAUNCHER function
From the docs:
category -- Gives additional information about the action to execute. For example, CATEGORY_LAUNCHER means it should appear in the Launcher as a top-level application, while CATEGORY_ALTERNATIVE means it should be included in a list of alternative actions the user can perform on a piece of data.
MAIN means that this activity is the entry point of the application, i.e. when you launch the application, this activity is created.
MAIN action is the main entry point of the application.
LAUNCHER category says that entry point should be listed in the application launcher.
MAIN : will start the activity home.This activity will be the Home page of the application.
CATEGORY_LAUNCHER : The activity can be the initial activity of a task and is listed in the top-level application launcher.