'All' refers to a complete list of all users on your site, including yourself. Within 'All', you can see all the different role types of your site, such as Site Admin, Instructor, Student, Affiliate, Lead, Assistant, and Support.
Go to Site > People > All
If you click on a specific role type, such as 'Affiliate', only the corresponding users with that role will be displayed. However, by clicking on 'All', you can see all the different role types and their corresponding users.
Also, watch the video on 'What Does 'All' Mean?