Brace height is the distance from the string to the deepest point of the grip with the bow strung. It controls how long the string pushes the arrow: shorter brace means a longer power stroke (faster, harsher, louder); taller brace means shorter (slower, smoother, quieter). Every bow has a range — typically seven to nine inches — published by the maker. Stay inside it.
Adjust by twisting the string: more twists raise brace height, fewer lower it. Work a few twists at a time, shoot a group, and listen. Most bows have an obvious sweet spot where they go quiet and group tight — find it, then measure and record the number so a new string can be set to match instantly.
Brace height also shifts effective draw weight slightly — a taller brace shortens the draw a fraction. This is one more reason to measure your actual draw weight at your actual draw length with a scale rather than trusting the number stamped on the limb, which is rated at 28 inches.