It's generally used to show how much you've contributed to the community. Since it's based on forum activity, the more of an active user you are on the forums, the more exp you will get (resulting in leveling up). Gaining exp is quite simple; you can post topics on the forums or reply to other topics to get exp. As you get higher in rank it'll take more exp to level up but if you're an active member and don't spam, you will rank up often enough.