World Locks can be bought in the Store for 2000 Gems. They have no rarity and are basically used as the in-game currency. They are mainly used for locking worlds though. One World Lock only can be placed in a world. Once placed, only you have access (only you can build). But by tapping the Wrench in your Inventory, you can tap the Wrench on the World Lock to adjust the settings. Which are: Adding/removing people's access (who can build there), allow anyone to build and disable custom music blocks.
World Lock

This is a World lock!