Hidden fields are questions that are only visible while editing the form as an Administrator. This question will not appear to other users (Applicants or Reviewers) anywhere on the form. Its job is to pass through a value such as existing information to the Administrator that you don't want to be visible to the user.
Within your form, you can use a Hidden Field in order to gather information about the user without having those details shown to the user accessing the form. An example of where Hidden Fields can be commonly used are within Budget Forms.
Using Advanced Expression Piping you can perform calculations within the form while not displaying the result to the end user. As an administrator, you can then use this hidden field within reporting or piping into other forms.