The NM represents the type of sheath on the cable. NM stands for Non Metallic sheath cable..

the rubber casing on the outside of the cable is what protects it from damage.

Cable sheath current is a current induced in the sheath by radio, magnetic or other interference, or a fault condition. The cable sheath should be grounded at one end so that this current flows to ground safely.

The metallic sheath prevents the ingress of moisture, while the armour protects the cable from damage from foreign objects.

Coaxial Cable