NFTs are tokens that can be use to represent ownership of unique items. They let you tokenise things like art, collectibles, even real estate and can only have one official owner at a time. They are secured by the Ethereum blockchain; no one can modify the record of ownership or copy/paste a new NFT into existence.
NFT stands for non-fungible token, Non-fungible is an economic term that you could use to describe things like your furniture, a song file, or your computer. These things are not interchangeable for other items because they have unique properties.
Fungible items, on the other hand, can be exchanged because their value defines them rather than their unique properties. For example, ETH or dollars are fungible because 1 ETH / $1 USD is exchangeable for another 1 ETH / $1 USD.