It seems obvious that grip pressure refers to how firmly your fingers hold the handle. That is true, but the way to get the pressure right is not to think about your fingers.
Grip pressure includes how the the hands press against each other, namely how pocket in the palm of the right hand rests against the thumb of the left hand. Contact must be secure, but without a feeling of the hands pressing against each other here.
For left-handed golfers it is the pressure of the pocket of the left hand resting on the right thumb.
The key point is to maintain that amount of pressure at that spot during the entire swing. That is very easy to do, and has the effect of keeping your fingers from squeezing when they shouldn’t.
While you are learning how to do this, pay attention at the places where pressure can easily change, which are at takeaway, at the start of the forward swing, or as the hands approach impact.