Plain English Meaning
A small adjustment that takes away the steady left or right pressure you'd otherwise have to hold on the controls to keep the wings where you want them.
Full Definition
A trim system that relieves a constant sideways (lateral) control pressure on the ailerons, allowing the airplane to maintain a wings-level attitude or a desired bank without the pilot having to hold pressure on the control wheel or stick.
Context Anchor
You encounter aileron trim when adjusting the airplane after takeoff, in , or any time the airplane keeps wanting to bank left or right even though you want it to .