When you edit a form and add a Single-line text field, you can choose to turn it into a hidden field as pictured below.
A page visitor will not see this field at all and they will not be able to write anything in it. It can, instead, be used to collect certain information:
Example 1: If you have more than one form on your page and want to know which one the visitor submitted from; You can create two identical forms, then add a hidden field called "form number" to each. The value can be "1" for the first form and "2" for the second form, then they can be inserted on the page accordingly.
This is how your visitor will see the form:
However, if you go to your leads dashboard you will see the following information:
Example 2: If you want to use Dynamic Text Replacement, either on a Thank You page or in order to track the source of the ad the visitor clicked on, then you can find the full guide here: https://d.pr/nYNJOA