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 preset by you on the back end 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 Data Entry Form, click on the edit pencil icon next to that field.
Next, click "add hidden field" and add the value for that field. Click "done" to complete and move on to add more fields.