What does [Sticky] mean?
It is posted before all of the moderators or administrators posts why?
A discussion topic that has Sticky on it, is a topic that stays at the top of the discussion stream (notice how it says popular as a heading on top of these stickied topics), so they do not get buried under many other ones. These posts were stickied, because of their importance to users on this website. Moderators and members of Duolingo can sticky a post that is very helpful to the community, so users can check it out.