What does 'out of submission storage' mean, please?

In a free account, the total submissions you could store is 500 submissions but you are having 578 submissions in your account.

You can take a backup of the submissions and then clear the submissions to clear the occupied space.

To delete the submissions, please refer to this guide:

Guide: https://www.jotform.com/help/377-How-to-Delete-Form-Submission-Data

Before deleting make sure you have taken a backup of the submissions as submissions once deleted could not be restored again.

For exporting submissions from each form individually, please follow this guide:

https://www.jotform.com/help/44-How-to-Export-Form-Data-to-Excel

For exporting only some submissions from individual forms, please follow this guide:

https://www.jotform.com/help/73-How-to-Partially-Download-Form-Submissions-as-Excel-CSV-PDF

For exporting all your forms, submissions, and uploaded files, please follow this guide:

https://www.jotform.com/help/374-how-to-export-all-of-your-data-at-once/

Let us know if you have further questions.

Thanks.