Blocks
Drupal themes are laid out in regions. 
These regions store blocks - the traditional building block of Drupal's non-page content. These blocks are custom content like links, menu sidebars, search forms, views of information, footer links, etc. The blocks are created in the editor and set to display in a region based on path or URL.
If I wanted a block of my contact information within the right-sidebar region of all about/[anything] pages based on the URL. I can go to blocks→ create block and enter in my content, set the display setting to right-sidebar and all pages under about/* and the sidebar will show up since there's content in it to be shown and my contact information will show. This works for some content but is exhausting to build out for when you have complex layout needs on pages.