I feel rating scales can be very different depending on the book you are reading however this is a rough guide to mine.
1 Star = A book that I DNFed.
2 Stars = A book that I finished but I had lots of problems with it.
3 Stars = A book that I enjoyed but still had some problems with.
4 Stars = A book that I loved.
5 Stars = A book that I loved and had that certain something that made it stand out.
I also have .5 ratings, for example I would give a book 3.5 stars to a book that I feel deserves more than 3 stars but isn’t quite good enough to get 4 stars.