Merit is a system by which you can not raise your rank higher than Jr. Member It can be obtained from other users. You need to write interesting posts, create new interesting topics and then maybe some of the users who have the highest rating, if desired, can reward you with mercury. With the help of a mint, you can raise your rating on this forum.
Here you can read more about Merit https://bitcointalk.org/index.php?topic=2818350.0