What are private newsletters?

This article explains how the Private feature in the newsletter screen works

In the Newsletters page under All letters, there's an option to make the newsletter private or public.

By default, the newsletter is set to public, and if you decide to make it private, you can use the toggle switch in the Private column. 

If you have been using Letterhead for a while, this feature used to be labeled as Public in earlier versions of our composer, and worked in the opposite direction.  If you moved the toggle to the right, the edition was Public, and default was Private.