If you view your Profile via your settings you'll see a section for "Recent Rep". Every member on the forum starts with 10 "rep points", and that number can only increase.
All rep is is a small forum feature that allows users to "give props" (positive only) to other users for a post they think is funny, well-written, well-thought out, etc. There is no criteria for why you should or could give rep to another user, but those are generally the reasons why.
The small green bar beneath your Forum Title will eventually grow the more rep you accrue, and the more you accrue the more "points" you can give out when you rep others in the process.
None of it has any meaning, really, and no one can see the rep you receive but you (and me), but it's a fun feature that can actually be useful.
For example, if you wrote out a great thread and someone thought it was a good idea, they can "rep" you with a comment like "Awesome thread idea!", and it'll show up in your recent rep, and you'll receive whatever the points value that member has at the time. The more rep you receive, the more you can give out.
Please note: There is no "update" for when you receive rep, unfortunately. The only way to check to see if anyone has sent you any is to view your profile via the Settings tab in the upper right corner. Your rep will show up in the "Latest Reputation Received" field.
• For every 365 number of days, users gain 1 point of reputation-altering power.
• For every 2000 number of posts, users gain 1 point of reputation-altering power.
• For every 200 points of reputation, users gain 1 point of reputation-altering power.
• User must have 10 posts before his reputation "counts" on others.
• User can only give out 10 reputations per 24 hour period, and you must rep 2 users before you can rep another again.