There exists a mechanism for creation of signature of any piece of data. You can extract an unique signature of an entire DVD, signature being just a handful of bytes long, DVD being gigabytes in length.
Two same DVDs yield the same signature. Change one bit on DVD, signature changes.
This technique is called hashing.
What NFT is, is a concept that you can "buy ownership" of something that's freely distributable over the internet, like a JPEG image, by having a record of buying its hash signature.
In total 5.yo terms it's like if a digital mona lisa gets released, even if the hi-res is paywalled, it will quickly be shared over the Internet for anyone with basic search skills. NFT thing allows someone to "own" the signature of the picture for reasons only known to them. It's a financial game alike crypto. They can claim assets. I don't know what the grand scheme is about, it's quite insane to me and really mocked in tech circles.
Furthermore, these nuts are pushing some sort of Web3.0 concept where everything is a microtransaction. There is a way to explain what it's all about and how it ties to NFTs and dystopian economy but it's 1 AM and that would be a long post.