It’ll get to a certain point in story and you have to select what the next action will be. I’ve tried not to read too much about it yet but here’s a section from an article I skimmed about how they made it -
“To begin with these are mundane – choosing between two songs or breakfast cereals – but they quickly get much more serious. Choices will send viewers branching off in different directions, only to sometimes jackknife back. Along the way, the system remembers past decisions, and these will influence things viewers see later in the episode.
Although most of the people behind the episode refer to it as a film, Weeks, who programmed computer games earlier in his career, likens it to a video game. As viewers make decisions they unlock certain parts of the narrative – a little like finding the key to another level in a game. If they die – and there is more than one way to die – viewers can re-spawn and replay the episode by making different decisions and unlocking different parts of the narrative.”