The difference between “active” and “hidden” keys is that “hidden” keys are not available to translators but are otherwise handled the same as all others. They are also exported to the output file.
Keys are automatically marked as hidden under certain circumstances - when the source language translation is empty or when they are linked to another translation using the Duplicity Linking feature available in the Agency Plan.
You can hide keys you don’t need to translate. Hidden keys will remain hidden until you reactivate them.
Learn more about the different states your source strings can be in in the docs article