This is surprisingly complicated for a Forum.
It might be unusual to link something like merits for posts to your rank on a forum--I've never seen it before--but believe me, it's necessary here. Don't know how familiar you are with bitcointalk, but a lot of members here are in signature campaigns and thus are paid to post, and as you would expect that attracts a lot of people from all over the world, desperate for money and who aren't necessarily qualified for any sort of writing job in a language they may not be proficient in. Most sig campaigns only pay for posts in English, and obviously that isn't everyone's first language. The result is that bitcointalk became flooded with garbage posts, many of which either don't add anything useful or just don't make any sense whatsoever.
And the higher your rank, the more advertising space you get in your signature, so we had people farming accounts and ranking them up either to sell or to enter into these campaigns. It got so bad by late 2017 that Theymos finally interceded and created the merit system, and now in order to rank up you need a certain amount of merits to get to each rank. Merits are given at the discretion of individual members if they have any to give, and the result is that now it's much, much harder to rank up. That's the history and a description of the merit system in a nutshell.
It's really not all that complicated, because it's basically a "like" function, except that the number of likes (merits) you can give out isn't unlimited. There are so-called merit sources, members who are given a monthly allocation of sendable merits (sMerits), and their purpose is to liberally merit good posts and thus allow members to rank up, although those merits certainly can be given to Legendary members who can't rank up any more than that.
It's a rough culture here, but you'll figure it out here if you stick around long enough.