A Permission, in Drupal, is an activity that can be performed within the site. Creating content, adding people to the site, and administering media are all examples of different permissions in Drupal.
Permissions are assigned to different Roles in Drupal. For example, a Site-Builder in OSU Drupal 8 has permission to both create custom content types and add new users to the site. A Coordinator in OSU Drupal 8 does not have permission to create custom content types but does have permission to add new users to the site.
The Permissions list in Drupal can become very long. Most modules have some type of permission associated with them, and some modules have many permissions.
Permissions can also be extended with modules such as the Group module - which has an entire subset of additional permissions that are used to manage members and content in distinct groups on a Drupal site.