What is the star shaped black thing that turns under the throttle side of the handlebar grip on a 95 Harley XLD?

Cruise control, using your thumb turn it to tighten, I have to use thumb and forefinger to loosen, hope that helps. It's just a simple friction lock, so hitting your brakes like on a car will not turn it off.

it is not a cruise control it is used to warm up the engine