NFT stands for Non-fungible token. It is a unique code that can not be copied, meaning whatever NFT you buy is unique to you and no one else can fake ownership of it. Usually you'll be buying (minting) uniquely individual digital artworks created by artists.
Some NFTs, like Tomorrowland's, allow owners to receive benefits (utility) in the form of digital/physical items and enhancements. Because you own the NFT, you are able to sell your NFT at any time. It is an asset worth money that belongs to you. All NFTs are purchased in crypto currency, because the NFT sits on the blockchain of a specific currency.
In short, you are buying and holding unique art pieces that will give you benefits with Tomorrowland in the digital and physical spaces. More information can be found on the Tomorrowland NFT site