A sticky, or sticky topic, is displayed at the top of the Message index, and is generally identified by a thumbtack icon, or some other distinctive formatting. Sticky is used to call attention to rules or important information that should not get displaced by newer topics from the first page of the message index. If you have permission to sticky topics, you will find a set topic sticky button at the bottom of the topic, and also a Sticky this topic check box in the Additional Options section when you make a post.