A hidden field can be added in use cases where you'd like to add a field that is hidden to the person filling out the form, but is shown to you on the backend to automatically add to each individual form fill.
For example, maybe you're at an event asking people for their home address. You happen to know that everyone at the event is from California so you don't want to spend time manually adding in that data each time a form is filled out. Add a hidden 'State' field with the value 'California' and each form fill will automatically add that field to the individuals' profiles.
To add a hidden field to your Form, click on field and scroll down to "Add Hidden Value."
Next, click "Add Hidden Value" and add the value for that field.
Scroll up the collection and click "Done" to complete and move on to add more fields.
The field will show that it is hidden as you are editing the form like it does here:
Proceed with creating your form and publish your form when your form is complete.