It's the product of the force and the distance at which it acts from the pivot.So 15lbs at 16" is 15 x 1.33' = 20
It's the distance between the centre of the axle of the motor (the pivot point), and the point at where the force is needed (the point where your motor's arm connects to the thing you want to move).
The normal way of measuring weight (what is often called "mass" though they're not the same thing) with a scale is by measuring force: the force at which the earth's gravity pulls at the object.So attaching a spring scale to your handle and pulling it is basically the same thing.
15 lbs of force on the handle that appears to be at least 16 inches from the pivot point? 20 pound feet of torque? Oh my, looks like a big gearmotor with an electromagnetic release clutch for manual operation.