Plain English Meaning
Trim is a way to set the controls so the airplane holds its current pitch, yaw, or roll on its own, so the pilot doesn't have to keep pushing or pulling on the yoke or rudder pedals.
Full Definition
An adjustment of small auxiliary surfaces (trim tabs) or equivalent mechanisms on the elevator, rudder, or ailerons that relieves continuous control pressure required from the pilot, allowing the airplane to maintain a desired attitude or flight condition without sustained input on the controls.