With the Phantom Pain, we all know the insane Metal Gear universe is about to get more insane. So for those who want to jump into Phantom Pain and have the slightest clue as to what is going on, check out this video from Eurogamer. It will explain everything you need to know about the Metal Gear series before taking on the newest version.
Keep in mind, this game is convoluted at times and even walking through it like this may still leave some things up in the air. It is Metal Gear, after all. Being weird and cryptic without pay offs is sometimes how this series works.