In mail template body and subject you can insert special tags called template variables:
In case of an email subject you must type it manually or copy the variable string from the email's content. They strictly depends on
They are dependent of an issue for which an user want wants to render template. They comes Template Variables come from available fields from selected issue type (so e.g. For example, when you insert field sub-task issue type may not have all of the variables present in a task issue type)
When you insert variable i.e. ${issue.reporter} into your email body the body's content, then Mail Task substitute will replace this value during sending en email from a specific issue. You can always preview your template variables added on subject and email body on your mail template in preview dialog for selected issue.