Hello Gail and congrats on using Buzz!
Enrollments are created when users are enrolled into courses. When a user is enrolled in a course Buzz creates a unique Enrollment ID that is associated with the user's enrollment in the course. Teachers are typically enrolled in courses to manage them while students are enrolled to participate in the class.
Subscribers when established allow for organizations to share courses/content to organizations that subscribed to their content. Essentially it allows organizations to share their content with other organizations.You would do this if you have reached out to another organization to share content with your organization.