Level Keys are rare objects that can occasionally be found in chests, left by The Keymaster, who is currently the only source of said keys, these keys can be used in The Hub to access other levels quickly.

The key to Level 1 can be found within your ender chest, and will only spawn once, being a gift from The Keymaster.

Another method of getting a key is requesting one from The Keymaster via his mailbox asking for a specific key for a level, which your request may or may not be fulfilled.