Enter the subject of your message. It is best to keep the subject short, simple and descriptive. The subject line is the easiest way for you and your correspondents to remember what was in the e-mail, and is generally used as the basis for quickly scanning e-mails in a mailbox.
When you "Reply" or "Forward" an e-mail, this subject line is copied over into the new message so that is becomes a valuable way of identifying which messages go together to form a "thread" to an e-mail conversation.
If a replied or forwarded message contains new ideas, you may want to alter the "Subject" line so that recipients can follow the progression of the "thread" and to indicate that the content of the thread has deviated from the initial topic.