I recently picked up a Mitchell Goldseries used with adjustable trigger. Using it is simple enough, hexkey screw in/screw out. What exactly does an adjustable trigger adjust?

It adjusts the over travel that takes place after the sear releases the hammer. Some people mistakenly think that it will adjust the weight of the trigger. Adjusting the overtravel screw incorrectly can lockup the trigger so it does not activate the sear or it will allow the sear to be "rubbed" by the hammer hooks.