A role is a setting that is linked to the user (and NOT to the content, that's the next section, Permissions).
In other words, your role will unlikely change anytime soon. It determines some of the activities you are allowed to do inside the Application.
The instructor role gives some elevated permissions, such as access to the classroom (connecting to students, seeing their screens, forcing content etc.), creating courses (and other content types such as tests and plans), hiding content from students / parents etc.
The student is the most basic role. There are no permissions that a student has that the teacher does not have.
A parent has the same (visibility) permissions as the student(s) they are linked to, except they loose two key things:
- All Edit rights (see next doc about Permissions) -- they can only see their student's content
- Membership visibility - they cannot see their students' classroom colleagues (for student privacy), only what relates to their own child.
The goal for this role is to be able to see what their students can see and to communicate with the teachers via direct chat.
What about administrators?
An admin is a teacher that has some admin rights (see Admin section). This is part of the customization of your school or district to set things up properly, as usage may vary based on your organizational structure.