Non-fungible assets are unique, irreplaceable, and non-interchangeable. Being non-Fungible means that individual items can not be mutually substituted without a change in value. Artworks, concert tickets, and diamonds are all examples of non-fungible items.
Let’s imagine for a moment that your friend Avis owns a famous painting like the Mona Lisa. A far fetched example to say the least. If your other friend Bailey offers to trade Avis a photo of the Mona Lisa for the real item, they would likely be laughed out of the room. Bailey could even offer to trade a painting they made, or any other piece of artwork. In any of the above cases an independant value would need to be assigned to both items as each individual artwork has unique properties, are irreplaceable, and non-interchangeable.
See also the definitions for Fungible and Semi-Fungible.