Form Fields are user-created fields that can be added to a template. They allow more customization with the data type, and field name, and more control over the field values.
Form Fields are different from Shared Fields; Shared Fields are used when the form is integrated into some external entity (ie: a proposal, or SedonaOffice Customer). Shared Fields retrieve the associated external entity values (ie: Customer phone, address, etc) and display them, they are not user-editable.
To create one, click the Form Field button on the left panel of the designer, and enter the Field Name (internal variable name for the field), the display text (name of the field shown on the designer):

Then the field can be dragged onto the form:

Form Fields can also be embedded within regular Text elements, by clicking "Add Data" and selecting the field:

On the top right, there are options to enable Sender/Recipient editing; I've enabled both Sender and Recipient Editing, but not required:

When sending this template, click the Assign button in the Form Details page:

Then on the Assign page, on the right panel, the sender can enter a value (some text in this example) for the form field:

After sending the form, and when the recipient goes to sign, they will see the value entered by the Sender, as well as edit it to add their own text:

For form fields embedded within Text elements, the values will be displayed alongside the text within that element, but is read-only:

Note: For the recipient to Edit the Form Field, they must be assigned to that field when sending the form. If not assigned, the form field will be read-only.