The set screw in a 1911 trigger adjusts the amount of over travel of the trigger.
These over travel screws are mostly a feature for a target pistol.
Over travel is the amount the trigger moves AFTER the hammer is released.
If there's too much over travel, the jerk as the trigger finally comes to a stop can cause jerking of the gun, and this degrades accuracy.
Not enough over travel, and the hammer will fail to release.
This is why many shooters who carry a gun for defense will not have an over travel screw on a pistol.
If the screw moves in, it can prevent the gun from firing when you really NEED it to.
To adjust over travel, turn the screw in until the hammer will not release when the trigger is squeezed with a normal pull.
Back the screw out 1/2 a turn at a time until the hammer will release, then back it out another 1/2 to 3/4 turns.
Apply Loctite to the screw to prevent it from moving any more.
Many people make the mistake of trying to adjust the over travel, "too close" and wind up having failures of the gun to fire."