Quota enforcement is not yet fully implemented. When it is, you will be blocked from adding further content.
Each subscription plan has limits on storage (uploaded files), and on the number of records. The current storage usage is shown on the Account - Plans page, with a visual indicator that changes colour as you approach the limit.
Enforcement of limits (blocking uploads or creation when you exceed the limit) is not yet fully implemented. When it is, you will see a warning as you approach your limit, and a clear error message if you try to go over it.
If you find that you are approaching a limit, consider upgrading to a higher plan, or deleting content you no longer need.