Unlockable content is additional digital assets or information that can be added to an NFT. They are usually not included directly in the NFT itself, but can only be accessed after the NFT has been purchased or owned. An example could be digital art where the buyer of an NFT gets access to a higher resolution of the art or another variant but also certificates of authenticity, invoice, confirmation of ownership.
Unlocking is usually done by using codes or keys provided to the buyer of the NFT. These codes or keys can be stored in the blockchain and can only be unlocked by owning the NFT.
Unlockable content can increase the value of an NFT by providing additional benefits or exclusive content to the buyer. They can also help the buyer view the NFT as something unique and special, as they are the only ones with access to the unlocked content.