Your audio storage is the total amount of hours of audio space you have available to use for the whole account, which includes uploaded audio files, published and not published episodes, imports.
It is an overall amount of storage that will not be renewed/reset monthly or annually but that can only be maintained by deleting old or unwanted content.
If you need more audio space you can also upgrade from here
Remember that you will get:
5 hours available with the Free Plan
100 hours with the On-Air Talent
500 hours with the Broadcaster
1,500 hours with the Anchorman
Unlimited hours with the Publisher
You can always check your available audio storage on the podcasts main section on the CMS.