Regions are represented in a theme's .info.yml file as key/value pairs where the key is the internal name of the region used to identify the region in code, and the value is the human readable name used in the user interface when identifying the region.

The content of a region consists of any blocks placed into the region that are visible on the page currently being viewed, and is generally output in a theme's page.html.twig template file inside of wrapping markup like a divintended to provide layout and structure.