NFT stands for non-fungible token. Simply put, it is a digital token that exists on the blockchain to record proof of ownership for the person holding it. Each token is completely unique because it has its own unique metadata which can never be replicated or replaced for another.
NFTs allow for proof of ownership in the digital world. Think of it like this, there is only one person who can own the real Mona Lisa, but millions can have copies of it. The value lies in the real Mona Lisa though. The same applies to digital collectibles and artwork. Before the invention of NFTs, there was no way to prove the ownership or authenticity of digital artworks or collectibles.