One doesn't simply jump from Metal Gear Solid IV to Metal Gear Solid V. There are important events that Hideo Kojima simply couldn't let go untold. Also, the Fox Engine needed a little warm-up before it is put through its full paces in The Phantom Pain. Basically, Metal Gear Solid V: Ground Zeroes is a prequel story leading into Metal Gear Solid V: The Phantom Pain. It features much of the same gameplay and a taste of the features of the main game, but in a smaller format. It's like pre-DLC, or even a demo, but don't let Konami hear you say that.
Metal Gear Solid V: Ground Zeroes stars Snake, and that's Big Boss Snake and not Solid Snake, as he infiltrates a Cuban camp in order to rescue several people -- people Metal Gear Solid fans will like recognize. The mission can be played multiple different ways, adding a ton of replay value to the game. That said, just a single run will be rather short. Hopefully everyone's willing to diversify their gameplay for the sake of getting as much value out of the game as possible.
Featuring that same ol' stealth third-person action, with Snake's pockets full of all sorts of fun toys and gizmos, Metal Gear Solid V: Ground Zeroes is the perfect option for those who just can't wait for Phantom Pain.