These fields exist in both this form and the mail template. When the form is submitted to the CGI script, it takes the values from this form and places them in the template. It figures out what goes where by matching the field name to the placeholder (a word in square brackets) in the template.
Square brackets have a special meaning in the mail template. They should only be used for variables and nowhere else.
Fields are not case sensitive. "Recipient" and "RECIPIENT" are treated the same as "recipient".