Help:Page protection
"Page protection" is a means of limiting which editors are able to edit a page. On most wikis it's used to prevent changes to certain important policy pages or to stop conflicts over editing public pages, but here on Shifti it's also helpful for authors who want to ensure that the stories they post are never vandalized or otherwise edited by others.
Protecting non-User pages
There are two main methods for protecting a story. One is to simply click on the "protect" tab at the top (or in the sidebar, depending on which skin one is using). By default users don't have this function available, but sysops and users who have been added to the "author" group have this ability. (If you have posted a story but are not yet in the "author" group, post a request on the user talk page of one of Shifti's sysops and you'll be added.)
When you click the protect tab you'll be taken to a page where you'll be asked to specify what level of protection you want and to confirm it. Select "sysop only"; even though authors are not technically sysops they have the same page protection abilities and so you'll still be able to edit the page after you protect it.
This technically allows members of the "author" group to protect or unprotect pages that belong to other authors, as well as other pages that have been protected by sysops. Please limit yourself to protecting your own stories, and respect the protection that others have put on their stories by not editing them without explicit permission. If you abuse your ability to edit other peoples' protected pages the ability can and will be taken away from you.
User pages and subpages
The other main method is to place your story on a subpage of your user page. Shifti only allows user pages to be edited by the user him or herself, though the associated discussion pages remain public. So, for example, the user pages User:Joe Bloggs and User:Joe Bloggs/Magic Cheese can only be edited by Joe Bloggs himself, but User talk:Joe Bloggs and User talk:Joe Bloggs/Magic Cheese are open to the public.