In the free option, what is meant by 100mb of storage?

How would one use this storage?

To the best of my knowledge, completed applications are sent to pvt email so they would't take up space? 

Please advise. 

A notification might be sent to your email if you want, but if a file was uploaded it will be saved on your Jotform account, and the email notification will only contain a link to that file. This way uploaded files are stored on your Jotform account and take space. Only by deleting the submission you will free up some space on your account.

You can see here the available space of each plan.

If you need further support please let us know.