Probably the most common question amongst new users!
Reputation or rep as most people call it, is how many upvotes a post gets. When you reach certain amounts you can unlock new features and rep titles. That's pretty much all it is, you can upvote any posts other than when they're in the 'Edge Of The Forum' category it'll just pop up with an error. Anyway, I hope I explained that thoroughly enough, if you have anymore questions though don't be afraid to DM me!