What are NFTs?
NFT stands for non-fungible token. Tokens like bitcoin and steem are fungible. If I send you steem you don't ask me "which steem did you send?" because they are all the same.
If I send you a Splinterlands monster you might ask "which one?" And if I say "Lord of Darkness" you might even ask again "which one?" and then I could tell you the exact ID of that one specific card because they are all different and uniquely tracked. When the tokens are all different like that they are NFTs.
aggroed has already explained in his post: