The Decor shop is where you can place decors buildings in Cookie Run Kingdom. These are purely cosmetic buildings, but completing a set of them will give you some bonus gems. This is also where you can find landmarks, extremely expensive buildings which give massive long-term bonuses, like increased attack, defense, and so on.
Sometimes your cookies will ask you to place decor and will reward you if you do so.
Also castle levels often have decor points requirements. Every piece of decor gives decor points, if you place enough of them the points will add up until you've met whichever requirement you're trying to meet.
See also: Resource Buildings