"She does not understand what I am saying."
Translation:Hun skjønner ikke hva jeg sier.
Skjønner means she understands or knows, but more in a spiritual way. You can "skjønner" a person.
Forstår is more for skill-like stuff. You "forstår" a language, for example.
In this example, she understands what you say as in...emphasizes.
If you were to use forstår, which could be right in this case, she would understand mentally. Maybe you're talking about how to write a paper, etc.