Plain English Meaning
It means setting the controls so the helicopter holds the attitude and heading you want without you having to fight the cyclic, collective, or pedals.
Full Definition
In a helicopter, trim refers to maintaining the cyclic, collective, pedals, and any installed force-trim or pedal-adjustment systems in such a way that the helicopter holds the desired attitude, heading, and flight condition with minimum control pressure from the pilot. Achieving trim means all forces acting on the aircraft are balanced so it remains stable in the chosen flight regime.
Context Anchor
Seen during helicopter instrument flying, especially when the , , and to keep the helicopter steady and .