Brace height is the distance from string to bow when the bow is strung, or braced.
When unstringing the bow, just slide the top loop down the limb. That way the string doesn't come off the bow and never untwists, changing the brace height. Adjusting the brace height may be necessary if the string stretches or settles in when it is new. Twist the string tighter to raise the brace height, effectively making the string shorter.