Teacher is one of the three core user roles in Moodle. The others are Administrator and Student. These roles control what you can and can't do within particular areas of a Moodle. In short, an Administrator can do almost anything and go anywhere, a Teacher has control over a specifc course within a Moodle and the experience of the Students that it contains, and a Student participates in a course.
It is recognized that the terms 'Student' and 'Teacher' may often be felt inappropriate and so the 'Teacher' of a course can choose terms such as 'Tutor', 'Facilitator', 'Trainer' and 'Participant', 'Learner', 'Customer' as is appropriate for their particular situation, with individual 'Teachers' able to edit their own title as they wish it to appear within a course (e.g. 'Visiting Academic', 'Librarian' etc.) . The terms 'Teacher' and 'Student' will not then be seen by anyone in the course of normal use. However, some decision had to be made about what to call these roles. 'Teacher' and 'Student' have been chosen, and this documentation follows the same convention.
The 'Teacher' (and 'Student') role is attached to a specific course in Moodle. Being a 'Teacher' in one course gives you no extra abilities in another, you must either be explicitly added to that course as a 'Teacher', added as a 'Student', or you will have no access at all.