When a collection uses the Galler Launchpad to issue their NFTs in the form of a Mystery Box, it means that collectors mint a NFT but don't know exactly which one they own until they uncover the box.
Collectors can decide to open the box and find out which exact NFT(s) they minted OR leave the box closed as purchased and re-sell it on the secondary marketplace.
The process to mint a Launchpad Mystery Box are exactly the same as a normal launchpad. The additional step you need to take is going to the NFT(s) you've minted and clicking on the Refresh button to get the associated metadata.
Please note that the Project/Collection selling the Mystery Boxes is in charge of uncovering the metadata and notifying their community that they can refresh the data and find out what they minted.