Forbidden Planet was an old 1950's SciFi movie with Leslie Neilson from Naked Gun (before he became a comedy actor).
To make a long story short, a scientist out in space found some ancient ruins that contained a machine that manifested an ultra-powerful monster from his own negative emotions.
Last season, a lot of people were guessing that maybe the scientists unleashed the psychic powers within Eleven, but she was SO powerful that the Upside-down plane (and everything in it) was just the manifestation of her own hatred of what they were doing to her. So that if she died, the Upside-Down and the Demogorgon would just cease to exist, because it never existed independently of her.
I never believed that theory myself, but it couldn't really be totally ruled out until this year. So, its not 100% true to say they really explained everything last season.