You might want to make a data field "hidden" in a form so you can track data that is important to you, but not important to the person filling out the form. 

For example, you might want to hide a "Lead-Source" field. You may want to know where the lead source came from, yet you don't want the subscriber to have to fill this information out.

After you have started the process to create a website form, you will find yourself on the Form/Content page that allows you edit your form details. 

Drag and drop fields from the Toolbox field on the left to the Form Layout. If there's a field you want to hide, click on the field and look at the Properties on the right; not all fields have it. If the field has a Make Field Hidden toggle, it will be at the top under Properties (Fig 1). 

Fig 1

This Make Field Hidden toggle is off by default; click it to toggle it on. When you're done with all your changes, scroll to the bottom and click Save if you're done editing the form or Continue if you'd like to advance to the next page in the Form Editor. 

Follow the next steps to complete the process to create a form.