Unlockable content is additional content you can upload when minting an NFT on SolSea that only the owner of your NFT can unlock. Unlockable content can be anything from higher resolution media, PDFs, BTS videos of your created art, other additional exclusive content, and even physical collectibles, prints, discount codes, access to private communities...
To mint an NFT with unlockable content, make sure you check the box when you upload your main artwork file.
Essentially, if you want to sell an ebook (PDF) but don't want anyone except the person who owns your NFT to have access to it, all you have to do is upload some kind of preview file (e.g. IMG or GIF) as a cover, and then upload your ebook as the main artwork file - don't forget to check the box for unlockable content while doing so!
When you mint that NFT, it will be marked with 'unlockable' and only the current owner of it can unlock your ebook.
To authenticate ownership of an NFT with Darkblock and unlock the content, you have to sign a message in your wallet (click approve in your wallet when a pop-up appears - you will not lose any funds, only confirm that you are the owner of the NFT).