...an NFT is just an entry in a ledger, controlled by a trusted entity. As an example; I own the property my house sits on. Why can't someone come along and just claim they own it? Because my local government's Land Registry holds a land title (i.e ledger entry) that says I own it, and the local government is a trusted entity (all jokes aside). If the government changes it's mind suddenly, scribbles out my name and says "Actually, nah, now that other guy owns it" despite the fact that I genuinely owned it before, then the government is no longer a trusted entity, but my claim's also invalidated anyway, there's now no record I own it. Alternately, maybe the Land Registry burns down and no record of my ownership exists anymore. This is an important point which I'll come back to.
Now, the Mona Lisa, well, that's pretty much like an NFT in it's own right. If you
have the Mona Lisa, then it's kinda hard to argue with. Conversely, if you set fire to the Mona Lisa, it ceases to exist... you don't own it anymore (incidentally, nobody owns it). But, if someone makes a damn good replica and says "That guy set fire to a fake, I have the real Mona Lisa!", the records of sale of the Mona Lisa held by a trusted entity such as (Not sure who, so let's say the auction company who sold it?) will show that person never owned it.
Now... thing is, the Mona Lisa is a painting on a canvas that physically exists. It was done like that because that's the medium of art of that time. Nowadays, while that still exists, we also have "digital artists" who can create some amazing work which requires just as much effort as traditional paintings. Of course, being digital, it's not like you can prop it up on an easle at an auction and sell it for hundreds of millions; sure you could print it and sell it, but the original work is the digital copy... who's to say you won't just print another and devalue the one you just sold? That's where NFT's come in... by placing a copy of the digital work and assigning it an NFT, controlled by a trusted entity such (such as a digital marketplace like OpenSea)... which now goes "Well sure, people might run around with screenshots or right-click-save-as entities of this digital work (just like how plenty of copies of the
Mona Lisa exist out there), but we hold the record that says this is the original, owned by this person". Hooray, now a digital artist can do a piece of art and be remunerated just like a proper artist. That sounds great right? What's the problem?
Oh lawd he comin'!
So there's a strong argument out there that most high-art sales are just financial scams. While I'd happily say #notAllArt, and there's some out there whose owners would genuinely believe it's worth a large sum of money, value is subjective with art; it has no intrinsic value. At the moment, the Mona Lisa was estimated to be worth ~ $100m USD in 1962, which translates to about $850m today. So if I buy the Mona Lisa for $2b, what's it worth? Ostensibly, $2b, if people out there want to pay that much to get ownership from me. And let's face it, there's a lot of people who would want to own it, whether for status, business (to display in a pay-to-access museum) or whatever.
But this is where the grift comes in.
Notwithstanding the destructive effects creating NFTs on the blockchain has, the "art" associated with it currently are low-effort pieces of, well, trash. But cryptocurrency offers a level of anonymity... so people will just make a bunch of trash art, "buy" it from themselves using a second account for a large sum, say, $10k, then people see "Oh wow, that sold for $10k, it must be worth it!" and then
someone with too many dollars and not enough sense goes and buys it (naughty words in link) thinking it's genuinely worth that. But then it gets much worse.
Like with the Mona Lisa auction house and it's provenance records, a peer to peer digital marketplace will hold those records as well. But there's several reports out there now of these places being hacked and NFTs "stolen", transferred into other marketplaces with a new owner, because hooray decentralisation right? Now, in the case of the Mona Lisa, if the records of that are destroyed, well, the advantage is I still physically have it... and if there's any doubt, forensic inspection will prove that. If another trusted authority now says someone else owns a digital asset, and the authority saying I owned it is no longer trusted, and I can't physically show ownership.... welll.... gg?
That's kinda just the tip of the iceberg that is this stupidity, but man, that's way longer than I expected. Just go with what Ian said.

FD shouldn't go near these things.