what is merit ? is it useful ? and how i can get some of them 
Depends. Are you here to rank up and join a shitcoin bounty to earn money? If you are, merit is the most useful thing of this forum. TLDR: Now you need merit to rank up. For example, if one day you want to become a Sr. Member, you will need to get 250 merits. You can get them by making high quality posts and getting merited by other users. This means that if you are a spammer, you won't get any and will be stuck in your current rank.
Better explanation: https://bitcointalk.org/index.php?topic=2818350
u can acquired your merit by just posting every 30 mins.thats what i have to do acquire merits
I would demerit you if I could