An example: If I trade oil, one barrel of oil with a certain spec[1] is fungible with another barrel of oil with the same spec. If I sell you some oil you don't care which specific barrel you get as long as it's the same kind of thing. Likewise if you borrow 10 dollars from me you can pay me back with any combination of notes up to the value of 10 dollars, you don't have to give me back the specific bills I gave you.
On the other hand, things like art are not fungible. If I loan a Van Gogh to a gallery I very much want the same piece to be sent back to me after the exhibition is done. It's not ok for them to send me something else. Even if it's "sunflowers" (since Van Gogh painted a few of those and they are quite similar), I want the actual one I loaned to be sent back to me.
Normal bitcoins or ETH are fungible whereas NFTs are non-fungible. The specific one you have matters usually because it represents a digital proof of ownership of some specific resource (a bored ape or whatever).
[1] The spec for oil types standardises particular grades and then people trade a standard barrel size (called a bbl which is short for "blue barrel" because they used to be painted blue). "Brent" and "WTI" are different grades. A barrel of brent is fungible with another barrel of brent but not with a barrel of WTI because that's a different grade.