So basically, every game comes with its own set of "unique" collectibles (various snowmen, plants, statues, etc), and to get them all, you have to share them with someone you know. If I share my items with you, you will get access to the unique items in my world and vice versa. There's two achievements tied to this function (one for receiving collectibles from another player, and one for building 25 unique decorative pieces).
Now, seeing as it's Christmas time, let's share the love and our collectibles with each other. My gamertag is "Vetle" (without the quotation marks). Send me a message via the Sharing-option in the pause menu, and I'll make sure to repay the favor. :)