aggroed has already explained in his post:

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.