CONTAINS SPOILERS

Datacubes store information about an item that can be unlocked and added to the pool of drops. This item will be attainable in future runs as well. First, you need to beat a boss. Doing so will give you access to the Datacube Processor. Deposit the Datacube into the Processor and you'll be given a consumable that can be used in your current run. It can appear in future re-runs as well.