One star ratings are short hand for DNF: 05/09/15
In my more than 20 years of tracking my reading each year, I've had many years where I've read more than 100 books. Since switching my web site's focus to book blogging, that number has been 300 or more. To keep that pace up, I have to have a lot of books going at one time and I can't always take the time to work through difficult books.
A difficult book doesn't have be one that has a ton of pages, a large vocabulary, or unpleasant subject matter. A difficult book is one that is difficult to come back to because reading it is a chore. That doesn't mean it's a bad book. It doesn't mean no one should read it. It doesn't mean that no one should love it.
But if a difficult book gets me to the point emotionally where I'm ready to set it aside and read something else, meaning it's a DNF (did not finish) then I give it a one star rating. The online sites I use as tools to track my reading, such as GoodReads, LibraryThing, or to release books, such as BookCrossing, don't have a DNF option for books. You've either read them or not. Or on some of them (BookCrossing, being the exception), you can delete a book from your virtual shelves.
Thus I use the one star as a a personal short cut for DNF. If I do finish a book but I still don't like it, I give it a 2 star rating.
As a librarian and book blogger, I still will recommend my one star books to you if I feel your reading interests are a good match for the book, or it's on topic for something you're interested in.