I believe this is a variation of the idiom "to grit your teeth" or "bite your tongue", they both in effect mean suppressing saying something, or suppressing anger. If you 'grit your teeth' (push them together) you cannot speak, so you cannot say something that you might regret or not want to say. Although the use of 'jaw' in the example is 'odd', I've not come across that particular usage before.

It would be more usual to say "she clenched her jaw." The meaning is clear. We react to stress in various ways. When we are suppressing anger, it is common to tense one or more muscles. Normally, when we are relaxed, our jaw muscles are working just hard enough to keep our mouth closed. To "clench her jaw" would mean to force the teeth together, as if biting down, tensing the jaw muscles and making them bulge visibly. To "grind her teeth" would mean not only to force the teeth together, but to move them back and forth against each other. When you grind your teeth, it makes a rubbing noise that only you can hear. To "grind her jaw" is interesting. It's a rare example but a Google search turned up other examples of use. As a native speaker I would read this without paying much attention to it. I think it means that you can not only see that her jaw muscles are bulging, but that you can even see some slight movement of the muscles. You can't see her teeth, but you can tell from way the jaw muscles look that she is grinding them. A good illustration of "clenched jaws" occurred during a handshake between U.S. president Trump and newly-elected French president Macron. . It is a perfect illustration of the word "tense" and "tension!" It seems to me that Macron clenches his jaw, and that Trump both clenches his jaw and grinds it. https://youtu.be/VOf9FqsLfA8?t=20