There's a full explainer here:
https://bitcointalk.org/index.php?topic=178608.0You get a new coin whenever you rank up (up to hero). You rank based on activity and merit and you can only receive 14 activity points every 2 week period (so there's no point making tons of posts if you've not got much too say but if you do have quality discussion or info then you might earn merit from it either way and may boost your account and rank there).