Plain English Meaning
It is a way of measuring the turning effect that a weight has when it sits a certain distance from a fixed reference point. You get the number by multiplying how heavy something is, in pounds, by how far it sits from that reference, in inches.
Full Definition
A unit of torque or moment equal to a force of one pound applied at a perpendicular distance of one inch from a pivot point. In aircraft weight and balance, pound-inches express a moment: weight (in pounds) multiplied by arm (in inches from the reference datum).
Context Anchor
Seen in aircraft weight-and-balance , especially when moments for passengers, baggage, , or equipment.