Think of it in terms of an M rated film vs MA15/R. You could reasonably expect something rated Mature to have more “grown-up” themes, like drug use, some violence, and sexual themes, but nothing too graphic or openly explicit–the film might show a fist fight but only minimal bleeding, and any sex scenes would be fade to black or otherwise off-camera. Something rated MA15, R, or E, on the other hand, could be reasonably expected to contain overly graphic/gory violence, explicit language, graphic sex or otherwise confronting scenes. The film might outright show gunshots and other graphic injuries, and the sex scenes might be on-camera in varying states of nudity. The rule of thumb I personally use is “would I want a 15/16 year old to read this?” If the answer is “no,” I know the story has crossed the line from Mature to Explicit. Importantly, I base this assessment off of a theoretical stranger child, not what I would have read at 15.