Module sections are the primary navigation for modules. They are defined in the module class:
protected $sections = [ 'books' ];
You can read more about section in the control panel documentation.
Sections can define buttons that allow you to take action within the active section.
Buttons are defined in the section's
protected $sections = [ 'books' => [ 'buttons' => [ 'add_book', 'view_books' => [ 'href' => '/library/books', 'target' => '_blank', ], ] ] ];
You can read more about button definitions in the control panel documentation.