In terms of YouTube, it's where you're offered to make money from your videos by allowing Google to place ads on them. The amount of money you make is determined by the amount of times the ads have been clicked or in s0ome cases watched. It's pretty simpl, but most people just starting out don't really make that much money if any. In my opinion, if you're just starting out (like a lot of us on here are) making money should be the last on your agenda.