The forum flairs and ranks are nice little rewards you can earn for posting on the forums and contributing to the community, and they are shown under your username each time you post.
Each rank has it's own threshold of posts you must meet in order to unlock that certain flair, however, once you go past that threshold, you enter a new threshold for the next flair, and will be stuck with it until you get to the next one, and you can't go back to a previous flair once you have the current flair and rank you have.
To find out what flairs and ranks require a certain amount of posts, please consult our Flair and Rank Database, which has every single forum flair and rank available;
viewtopic.php?f=34&t=4755
Please note: Do not post randomly to increase your post-count to get to the next rank. Your posts will be removed for spam, and you will be warned accordingly. For more information, please see Rule 7 of our forum rules; viewtopic.php?f=11&t=67198