Simply put, trigger reset is the point in a triggers return (after a shot has been fired) at which the gun's internal mechanisms are ready to fire again. Most semi-automatic pistols provide an audible "click" which you can sometimes hear or feel in the trigger. This "click" lets the user know that the pistol is ready to fire again.
Instead of letting the trigger come all the way forward after a shot, train yourself to keep the trigger pulled to the rear and slowly ease it forward until you feel or hear a "click". At this point, pull the trigger again. You should notice that there is almost no pre-travel and that the effort and movement needed to break the trigger is greatly reduced. This reduction in pre-travel and movement allows for extremely fast and accurate follow up shots. Most people will experience a major increase in accuracy when they start "shooting from the reset".
With practice, this method of shooting will become habit and you will be one step closer to mastering the GLOCK trigger.