Thanks! The curation rewards have changed recently so most earlier posts on the topic are now incorrect. But basically if you upvote a thread when its still new, and it goes on to make a ton of money, you get a portion of that.

I think 25% of the payout of each post is given back to the curators. So the more the post makes the more potential profit for giving it an upvote. Also, the more steem power you have the more of the payout you get. So at first you dont get much, but over time it gets better and better.