Each course can have a Forum for the teachers in that course. This is especially useful when several teachers collaborate on a course. To create a teacher-only forum, add a normal forum and then hide it. Teachers can view hidden course activities while students cannot.

If you are the course creator or a teacher in the course, when you enter the Teacher forum you see several links in the upper-right corner:

The line Everyone can choose to be subscribed means that right now all teachers in this course can choose to be subscribed to the forum or not. Subscribers receive email notices when someone posts to the forum. Clicking on this link forces all teachers of this course to be subscribed to the forum. It is one of Moodle's quirks that not only can the course creator force all the teachers to be subscribed, but so can any teacher of this course.

The links for Show/edit current subscribers and Unsubscribe from this forum do exactly what they say.