What is hidden value fields

I am not sure what you mean by "What is hidden value fields".

You can hide form fields and you can assign calculation values in the options of some fields.

Related Help Articles:

How-to-Hide-Form-Fields

How-to-Assign-Calculation-Value

If you meant something else, please post a screenshot of what you are referring to here.

How-to-Post-Screenshots-to-Our-Support-Forum