A nylon hex nut can mean either a fully plastic hex fastener or a metal nut with a built-in nylon locking ring (a Nyloc nut). Pure nylon nuts provide electrical insulation and rust resistance, while nylon-insert lock nuts resist loosening from heavy shaking and vibrations.

No Rust: Pure nylon does not rust or corrode in wet places.
Vibration Proof: Nyloc rings stop bolts from backing out over time.
Safe on Surfaces: Plastic nuts will not scratch or damage soft parts