Leanback, a graduation of YouTube's XL format, loads up either videos that arrive through your subscriptions and YouTube preferences or videos your friends are sharing on FaceBook. If you're not all that hooked up on YouTube, you'll see a selection of clips that are proving popular. Clips seem to arrive in HD format by default, or perhaps depending on your connection speed.
Here's how Google explains Leanback in video form, with instructions on hooking up your Facebook account.
YouTube Leanback offers effortless viewing [YouTube Blog]