Something fungible is able to replace or be replaced by another identical item; fungible things are interchangeable.

Dollar bills are fungible - I can exchange one for another. Snickers bars are fungible. If I buy a Snickers bar, I don't care which one I get.   The Mona Lisa is not fungible. If I buy the Mona Lisa, it better be that particular painting.

Non-fungible tokens are iffy digital "currencies" which the holder cannot freely trade or cash in. A chunk of such a currency can be called a token. Probably the owner of such a token can do business with other owners of tokens, but not with outsiders. In contrast, I can exchange U.S. dollars to Swiss francs, but I can't exchange certain (or maybe all) cryptocurrencies for other cryptocurrencies.
That's why they are non-fungible.

Forgive me if I don't know all the ins and outs, because I wouldn't touch these deals with a ten-foot pole.
   
There are deals going on where buyers purchase art which is only stored digitally, using non-fungible tokens. Apparently the music of a band called The Bored Apes is one of those kinds of art. If you're interested, you pay with a non-fungible token and you become owner of a piece of music which only you can access. Unless somebody hacks it, I guess.

To me, it's amazing that after centuries of prosecuting counterfeiters, the governments and central banks of the world stood around and looked dumb while unknown people developed vaguely-defined money called cryptocurrencies.