The level of peoples profile is simply gained by posting around the forum. The more posts one have the higher the profile level.
This list is old and not 100% accurate on higher levels since it's quoted from an old news thread, but the idea is still the same. I do believe it's 5000 posts for legendary instead of 4k as listed below.
Newbie * 0
Jr. Member * 25
Sr. Member ** 50
Experienced Member ** 100
Dedicated Member *** 250
Full Member *** 500
Elder Member **** 1000
Elite Member **** 1500
Hero Member ***** 2500
Legendary Member ***** 4000
It's really there more for fun, and something common to have on forums to encourage users to post and be active. Counting away the first level that is 25 posts and that it allows you to nuzzle users, the levels do not really change anything at this point in time.