An NFT or non-fungible token is essentially a unique digital asset that cannot be replicated. The ability to create collectible and rare digital artwork relies upon this blockchain technology to identify that each artwork is authentic and therefore ensures that collectors are buying into a digital art marketplace that has real value in the long term.

Or put more eloquently by Cryptoart expert Jason Bailey (Artnome) :

CryptoArt are rare digital artworks, sometimes described as digital trading cards or “rares”, associated with unique and provably rare tokens that exist on the blockchain. The concept is based on the idea of digital scarcity, which allows you to buy, sell, and trade digital goods as if they were physical goods.

Jason Bailey – https://www.artnome.com/news/2018/1/14/what-is-cryptoart

The following link to coinmarketcap also explains what a non-fungible token is in more detail. https://coinmarketcap.com/alexandria/article/what-is-a-non-fungible-token-nft