Plain English Meaning
How hard you push on a rudder pedal with your foot to swing the airplane's nose left or right.
Full Definition
The force a pilot applies with the feet to one of the rudder pedals to deflect the rudder, controlling the airplane's yaw (nose left or right) about its vertical axis. The amount of pressure -- not the distance the pedal travels -- is what pilots are trained to feel and modulate.
Context Anchor
Seen in discussions of airplane stability and control, especially when describing how much pedal force is needed to keep the airplane pointed where the pilot wants.