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 email, thus is very important in organizing your email and finding important ones later.
Also, when you "Reply" or "Forward" an email, 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 email conversation. As a suggestion, if a replied or forwarded message contains new ideas, you may want to alter slightly the "Subject" line so that recipients can follow the progression of the "thread".