Regions

Regions play a critical part in theming, as Drupal needs to know exactly where content can be displayed. This has an impact on what regions are visible to the Block layout for both system blocks and custom blocks that we may want to use. If we do not specify any regions within our *.info.yml file, then Drupal will provide us with regions by default.

Note that if we decide to add additional regions to our theme, we must also add the defaults or else we will not have access to them. Also, any regions that already have content assigned to them that have been removed by our theme will have the content disabled on the Block layout page.Default Regions:

regions: 
  header: Header 
  primary_menu: 'Primary menu' 
  secondary_menu: 'Secondary menu' 
  breadcrumb: Breadcrumb 
  help: Help 
  highlighted: Highlighted 
  content: Content 
  sidebar_first: 'Left sidebar' 
  sidebar_second: 'Right sidebar' 
  footer: Footer 
  page_top: 'Page top' 
  page_bottom: 'Page bottom' 

The value for each...