Can someone please explain me what is a sub-user ?

I saw that on the starter pack, there is only one sub-user. What does that mean ? 

Sub-user accounts are where you'd like to give other users read or view access to some or all of your forms. When you grant users access they can log in with a separate account and access the forms and submissions that you've selected for them. 

If you want to know more about creating a sub-user then please check this guide: https://www.jotform.com/help/232-How-to-share-forms-with-a-sub-account-user

Hope that helps. If you need anything else, please contact us again. Thank you.