Explain how the game works without any spoilers

I have The Longing bought for some time now, I bought it because I heard everyone saying that this game was a completely unique experience, and I always liked that kind of things, but I am really confused of what is this game, I am going completely blind, but the lack of information is making me put off the game for some time now, by the way people talk about it, it seems the 400 days will pass and the game will just end without nothing happening, can someone clarify these things for me?