Your Capitan License provides your website with a disk space allocation, which is much the same as a storage on a mobile phone. It's the amount of data which can be stored on your website.
If your website exceeds the allocated diskspace you'll need to upgrade your subscription to add more space, or remove some old files.
Information about allocated disk space can be found here:
Check allocated disk space for Capitan Individual License
Check allocated disk space for Capitan Enterprise License
Check allocated disk space for Capitan Charity License
Information about increasing your disk space can be found here:
How to increase my allocation. Booster: Disk Space
How to check your allocated capacity and usage:
Login to Capitan and to to "Settings", you will find your license usage and capacity on the right hand side of the page.