It's the screw on the bottom front of the grip frame that tensions the mainspring. It's supposed to be fully tightened. Loosening it or using a shorter strain screw will lighten the trigger pull but could also cause light primer strikes. Additionally, if you are loosening it on purpose, it can loosen even more after repeated firing and cause light primer strikes.
Keep it tightened so it won't back out on it's own and keep it at factory specs.