An NFT is a piece of digital artwork that only exists in the virtual/digital world, not in the physical world. It is defined by a digital file or token. Like crypto, an NFT can be bought and sold. So fiat money (like dollar, pound, rupee, yuan, etc.) can be converted into crypto (digital money like Bitcoin, Ethereum, Solana, etc.) and which in turn can be converted into an NFT, or from NFT to crypto and then back to fiat.
If an object is fungible, it can be swapped for something else that is of equivalent value. Think of a physical world item that you can touch/feel like a $50-dollar bill, for example, can be swapped for other dollar bills (i.e., two $20s and one $10). So does it mean all dollar bills are fungible (exchangeable for other denominations that hold the same value)? If a $50-dollar bill is unique/rare (i.e., repeating digits in the serial number, or signed by a famous person, etc.), then the $50-dollar bill holds more value so it is not exchangeable for another $50 bill. Therefore, a unique/rare $50-dollar bill becomes non-fungible.
Like our rare $50-dollar bill example, an NFT holds some unique/rare properties that make it valuable. Although a digital file representing an NFT could be resaved by, for example, taking a screenshot, that does not change the owner of the NFT or diminishes the value of the NFT. Similarly, if someone takes a picture of a rare (non-fungible) $50 dollar bill, that does not diminish its value or change ownership.