An open is recorded when the images within an email message are displayed. Not every email client displays images by default, so the open statistic is never an exact count of the people who have viewed your email. However, the open statistic is most valuable when viewed in context - in comparison to previous mailings to the same audience. For instance, you might use the open statistic to judge the quality of your subject line, or to decide which day and time is better for sending your email.
To improve your open rate, make sure you have a good mailing list that wants to receive your messages (opt-in), use a familiar FROM address and include a relevant subject line.
As email bounces or people unsubscribe, the total number of delivered emails will go down (assuming the list is not growing in other ways). The open rate of a smaller list with people who want to receive information from you is generally higher than a larger, less targeted list.