Plain English Meaning

Trimming is the process of fine-tuning the airplane so it flies hands-off in the attitude and speed you want, instead of you having to push or pull on the controls the whole time.

Full Definition

The act of adjusting an aircraft's trim controls so that the flight controls require little or no sustained pressure from the pilot to maintain a desired flight attitude, airspeed, or configuration. Trimming relieves the pilot of holding control pressures by repositioning a small auxiliary surface (or equivalent system) so that the stays in the the pilot wants without continuous input.