A region is an area within a theme where blocks can be placed. Typical regions include header, footer, sidebar first, sidebar second, and content. The theme defines which regions exist through its info YAML file.
When you visit the Block layout page in the admin interface, you see a list of regions with the blocks currently assigned to each. You can drag blocks between regions, add new ones, and control their ordering.
Regions are a theme level concept, which means changing themes can mean rearranging blocks if the new theme defines different regions. Good themes keep their region names consistent with community conventions to ease theme switching. The region system has been around since early Drupal versions and remains one of the platform’s most flexible layout mechanisms.