A role is a collection of permissions defined for the whole system that you can assign to specific users in specific contexts. The combination of roles and context define a specific user's ability to do something on any page. The most common examples are the roles of student and teacher in the context of a course.
More on roles and permissions on the Moodle website (may be available in English only)
Enroll a user
Steps: Course administration > Users > Enrolled Users
- Select Enroll users.
- Select the role from the menu.
- Search for the user.
- Select Enroll.