Newton-meters (Nm for short) and foot-pounds (ft-lbs) are units of torque, also called the moment of force or the rotational force. Torque measures the tendency of an object to rotate if you apply some force to it.
Imagine an object, such as a lever, that has one fixed point. We will call that stationary point the pivot point. If you start pushing or pulling the lever in a straight line, the object will begin to rotate around that pivot point. The further from the pivot point you apply the force, the bigger the rotation.
The product of the force you apply and the distance from the pivot point is called torque. If you're a fan of the SI unit system, you will probably measure it in newton-meters. If, however, you use the FPS (foot-pound-second) system, you might be more familiar with the unit of foot-pounds.
💡 Remember that foot pounds and pounds-force are not the same unit, although their symbols are quite similar. You can express torque in foot pounds (ft-lbs), but not in pounds-force (lbf) - that is a measure of force.