Cinemascore grades the audience’s reception towards the film. The higher the score, the more positive the reception.
For family films, you want to get an A in order for a film to do well in the box office. A- is passable, but a B-range is not good since that means most of the people within a certain age range and/or gender must’ve had mixed reception.
The same is true for superhero films. You want to land at an A-range for those films, because if you don’t, then something did went wrong with the film that disappointed the audience.
For horror films, you want to land at a B-range. Horror usually tends to get a lower reception than most films, and a horror film getting a Cinemascore of A- or higher has been extremely rare in cinema history.