Switches between showing fields as field names or field values. When enabled the field names are displayed, and when disabled the field values displayed. Some field contents cannot be displayed.
To change the default field display to field names instead of the field contents, choose LibreOffice - PreferencesTools - Options - LibreOffice Writer - View, and then select the Field codes checkbox in the Display area.
When you print a document with View - Field Names enabled, you are prompted to include the field names in the print out.