A Permission in Revolution is simply a single access control that allows you to deny or allow access to a task. They are collected in Access Policies into a list, which is called an Access Control List (or ACL). From there, the Access Policy defines all the Permissions that the list contains.
An example Permission is "content_types" - if the Policy does not contain this Permission, then users with that Policy will not be able to view the Content Types page.
Permissions are assigned in Policies in access control lists. To do so, simply edit any available Access Policy, and from there you can view and manage existing Permissions, as well as create new ones.