Rebuilding may take some time if there is a lot of content or complex permission settings. After rebuilding has completed, posts will automatically use the new permissions.
Number of posts on main page: provides a dropdown box that determines the default maximum number of posts to display per page on overview pages such as the main page.
Length of trimmed posts: provides a dropdown box that determines the maximum number of characters used in the trimmed version of a post. Drupal will use this setting to determine at which offset long posts should be trimmed. The trimmed version of a post is typically used as a teaser when displaying the post on the main page, in XML feeds, etc. To disable teasers, set to 'Unlimited'. Note that this setting will only affect new or updated content and will not affect existing teasers.
Preview post: provides two radio buttons that determine whether it is optional or required for users to preview their posts before submitting.