The people shown as instructors on a course page.
An instructor is not an admin user
An instructor profile is what visitors see: a name, a bio and a picture. It is not an account and grants nobody access to the admin.
If you want an instructor to actually edit their course, give them an admin user with the right permissions — the two are separate jobs.
A course can have several instructors
All of them are shown on the course page.
Deleting an instructor does not delete courses
The course stays and is simply shown without them.