A program offered by YouTube which has many perks, such as your video being promoted by YouTube themselves, the ability to make a banner for your channel and customize it more than a non-partner's channel, and more recognition by the YouTube community as a whole. You also get paid by YouTube to sell out by advertising products in your videos.
Person A: "Dude, Smosh used to be awesome, but now they're just sellouts."
Person B: "What do you expect? They're YouTube Partners, they get paid by YouTube to advertise products in their videos."