NFT
An NFT is a non-fungible token. It is a blockchain record tied to one specific digital item. Unlike bitcoin, one NFT cannot simply be swapped for another one like it: each has its own value.
The price of an NFT depends on the asset attached to it and can swing hard. The token proves an ownership record, not always the rights to the file itself.
Example
Two ETH are the same, so you can trade them one for one. Two NFTs from the same collection are not: one is rare, the other common, and they sell for different prices.
What not to do
Do not buy an NFT because of social media hype, and do not open links to a "free giveaway". That is a common way to lose access to a wallet.