Sticky topics allow forum topics to be listed at the top of a forum even if they don't have the most recent update.
To be able to use the sticky topics functionality you need to make sure the top page of the forum hierarchy has been designated as a forum root page. You will also need to have permission to set sticky topics.
To make a topic 'sticky' click on the make sticky button under the tools menu for that page. If a page is already marked as being sticky, you can remove this property by clicking on the remove menu item in the same location.