If logged in, the visitor can view all the pages/posts on the blog. If not logged in, the visitor can only view a specific public page.
Almost Users Only is a plugin that limits public access to your blog. Visitors that are not logged in can only view a specific public page. If they are trying to reach other pages they will automatically be redirected to the public page.
If a visitor is logged in, he can view all the pages/posts on the blog (just like if it were a normal WordPress installation).
The public page can for example be used to display information about the site and a guide to how visitors can get an account.
IMPORTANT: WordPress still publish RSS feeds, so posts can still be read that way. Pages should be save though.