A shelf is a decorative item that looks like a shelfy bit of horizontal wood. You can find this item in the furniture section of inventory. Despite their real-world purpose of holding things up, in-game shelves do not have collision.
How to Obtain
Although you can mine shelves in holiday arctic worlds, where they generate naturally in specialized gift box bunkers, it's easier to simply craft or trade for them. Shelves themselves are not further used in any crafting recipes.
How to Use
Use shelves to make it look like you are storing various items off the ground. Or put a couple on either side of a crane base to create a helicopter rotor.