The idea is that a developer becomes an affiliate from Apple. Each time he’ll bring people to the App Store, he’ll get a chance to make money. The be precise, he’ll generally earn 5% from everything people will buy during the 72 hours after being sent there. For instance a user click on the affiliate link of my app, doesn’t buy it, but decides to buy another app on iTunes that will cost $20, then I’ll earn 5% x 20 = $1.
For detailed instruction on becoming an App Store Affiliate,