Rogue One: A Star Wars Story

I didn't know there was another Star Wars movie coming out this year. Rogue One: A Star Wars Story is set to be released this December and it tells the story of how the Death Star plans were stolen.

The film is part of a series of planned spin-offs based on the Star Wars universe, but not directly involved in the telling of the epic space series. It's confusing, kind of. Somebody please explain.