It states that given the size of the universe it is not improbable to assume that any number of branching timelines could exist at the same time, making it so that any given reality could be just as real as the other, simultaneously existing and not existing at the same time, existing in its own reality but not in that other reality. It isn't essential to play Ground Zeroes to enjoy The Phantom Pain, but it serves to tease the main plot, allows players to get a feel for the darker tone, and as a refresher on the Big Boss timeline.
The main story mission makes up 10 percent with the rest being divided among the side ops, extra ops, and trials. 100% Completion: Your completion ratio is given as a percentage at the title screen.