The Realm of the Primes is an ethereal plane beyond time and space, and inhabited by several members of the original Thirteen. Some of them, such as Micronus, are capable of altering the realm to their will, reshaping its geography or creating illusionary constructs.
After his fellow Primes fell to infighting and betrayal, Vector Prime lost hope and sealed himself away in the Realm of the Primes, a private realm outside of space and time.
After gathering all of the Blades of Time and properly arranging them, Optimus Prime ended up in the Realm of the Primes, where he encountered Vector Prime. Vector Prime theorized that time moved differently in this realm compared to the rest of the universe, but upon returning to normal space Optimus was relieved that no significant temporal dilations had occurred
Before becoming one with the AllSpark, Optimus was rescued by the Primes and brought to the Realm of the Primes, where they began training him to face an impending threat, though the Primes allowed him to appear in the material world when Bumblebee's need was dire.
Micronus was able to create a hill and manifest humans and a robot foe for Optimus to fight.
Optimus continued to train under Micronus's supervision. When Megatronus threatened to reappear, Optimus Prime was empowered by Vector Prime and subsequently released from the Realm, causing him to rematerialize on Earth.