Marcia,
Are you using advanced forums?
If so then this is specific to advanced forums and not part of the Moodle core forums. The flag is supposed to represent a bookmark as in you are flagging the post for later review. The s is to mark the post as substantive. The s gets tallied in Joule grader when manually grading a forum and the bookmark is at best a visual queue in the tree view. We never implemented a feature like forum ng where there is a separate area that shows just bookmarked posts.
See http://kb.moodlerooms.com/article/8/teacher for full documentation on advanced forums.