A title or brief description of a message or document. For example, in an e-mail, the subject is used to describe the contents of the e-mail. A subject, such as "display problems," may be used to describe that the message contains information about a problem with a computer display.
Whenever sending an e-mail you should always include something in the subject line, even if the subject is one word. A subject helps recipients quickly identify what your e-mail is about and may even cause them to open the e-mail faster when viewing dozens of unopened e-mails.
As may be seen in the image below, the subject is the fourth option listed in this example.