A merit can be compared to "like" of face book, twitter and other social medias. But the merit points reserved for us are limited. We can give merit to the person who has a great contribution to the forum such as producing quality post and comment. 
Can't believe that there are still some poeple who can't understand the real purpose of Merit after implementing it a couple of months already. I would rather agree to @franky1's explanation comparing Merit about YouTube's views/likes than this one.