what is 3 sub-user acount of premium (upgrade)?

Now, i have had upgrade from free to premium. I have 3 sub-user account also. Tell me What is it??? How can I use them?

UPDATE: Effective October 2018 the Sub-User feature has been discontinued and is no longer available for creating new sub-users. If you had been using this feature previously, you're grandfathered. 

 If you would like to have additional users added to your account, please check out JotForm Enterprise.

The 3 sub-users are 3 additional new or existing usersr on free or paid status. They can be added to just view the submissions of one or more forms or they can even edit one or all forms in yor account

Here is a detailed guide on how to

https://www.jotform.com/help/232-How-to-Share-Forms-with-a-Sub-Account-User