In short Non-Fungible Token — a unique on-chain certificate of ownership for digital (or physical) items: art, collectibles, tickets, domain names.
What the chain stores is usually a pointer, not the artwork — the image itself often lives elsewhere, so an NFT is only as durable as where that file is kept. Beyond collectibles the format is quietly useful wherever "this specific one belongs to you" matters: event tickets that cannot be duplicated, domain names, memberships, in-game items. Judge each one by what the token actually entitles you to, which is a question about the contract and the issuer, not about the picture.