Private stories are stories in which you would like to regulate who reads them. When a story is marked as private, only your followers are able to access it if they have a link to it.
To mark a chapter as private, it must be a draft. When editing your work, select the Advanced option in the right hand column. At the top of the advanced options, select Private to mark your chapter as private.
Note: Once your chapter is made private, you cannot change it to public, you'll have to submit a request to change it back to public.
Check out this guide for more information on reading private chapters.