It's an overtravel adjust screw. Cranking it in (CW) reduces the distance that the trigger can move back. Eventually, it gets to the point where the hammer won't fall. Before that, it goes through a region where the hammer spring is strong enough to force it to fall and it tears up the sear nose as it drags past.
The way to properly adjust it is to take the slide off, remove the mainspring housing and turn the screw in VERY slowly. After each change, pull the trigger all the way back and use your thimb to gently move the hammer from "cocked" to "down". If you feel it touching the sear, back the screw out far enough that the hammer can move freely past the sear when the trigger is pulled all the way in.