Quite exactly what it says it means. It means you posted your first message on this forum and for that you have won a trophy. Trophies mean basically nothing though. It might be your message was made some time ago but due to the transition to XenForo it looks like you just made your first post.
To just quickly add onto what Wortel said, you can view other trophies here: http://www.sythe.org/help/trophies
They are milestones that only you can see.