Plain English Meaning
Using the trim wheel or switch to take the constant push or pull off the control yoke, so the airplane will hold its nose at the angle you want without you having to fight it.
Full Definition
The act of adjusting the elevator trim control to relieve continuous control-yoke pressure required to hold a desired pitch attitude during a phase of flight such as takeoff, climb, cruise, or descent. Proper elevator trimming sets the trim tab (or equivalent trimming device) so the airplane maintains the selected pitch attitude with little or no sustained - or forward-pressure on the yoke.