The Flesch-Kincaid score determines what grade level you are writing. If your score is 10, you are writing at a 10th grade reading level. If your score is 12, you are writing at a 12th grade level. And so on.
The F-K score is calculated by words per sentence (lower is better), syllables per word (lower is better), and a few other factors.
Good sales writers aim for as low a level as possible. Anything greater than 8 is considered bad sales writing. People get fired over it.