The Stanley Parable is an experimental narrative-driven first person game. It is an exploration of choice, freedom, storytelling and reality, all examined through the lens of what it means to play a video game.
You will make a choice that does not matter
You will follow a story that has no end
You will play a game you cannot win
...it's actually best if you don't know anything about it before you play it :D
I REALLY encourage you not to watch this trailer and instead to simply play the game without knowing anything. That said, if you desperately need to know a little something, give this trailer a look for a sense of what kind of mood the game sets.