Fungibility is an economics term that describes the interchangeability of certain goods. For example, a barrel of oil is fungible (interchangeable/indistinguishable) from any other barrel of oil. A dollar bill, likewise, is equal to any other dollar bill (or 4 quarters, etc.). Non-fungible is to render such items unique or distinguishable. For instance, if you were to take a dollar bill and have it drawn on and signed by a famous artist, it become unique - unlike all other dollar bills, and perhaps worth more than its face value.