The Rare Orb is a mystical round Crystal Ball that holds the power to open the door to the Golden Temple. All eight of them are required to activate the doorway.
To collect rare orbs you must complete all levels in a world with all the K-O-N-G Letters. Once you have completed each level with the K-O-N-G letters, a new level will be revealed in the world. Completing this bonus level will earn you a Rare Orb. The bonus level is also called a temple.
Temples are extremely difficult. Rare Orbs are held in a shrine at the end of a temple. The temples are made up from difficult platforming, to vertical levels, to the classic barrel shooting levels.