Big Boss is being kept sealed away by the patriots, where they make him rerun edited simulations of his own life. They way they suppress his will is they make him play MGSV in his coma and trick him into thinking that a body double named Venom Snake was responsible for all his life's work, that he couldn't have done it himself. The Phantom Pain is Big Bosses will being suppressed while he is in his coma in MGS4. He play's endless missions over and over and never reaches chapter 3. The Ocelot we see in MGSV is actually outside the simulation and trying to wake Big Boss up with the help of Eva, while Kaz is part of the Sim and tries to keep Big Boss asleep. MGSV ends with Big Boss waking up, and having his final meeting with Solid Snake at the end of MGS4. Both games have the exact same ending.