Email Beacons are also known as invisible GIFs, 1x1 GIFs or other names. They are used to inform the sender about each time the receiver opens the email.
The email contains a link to an image, often invisible to the viewer, that the email client or browser will obtain from a web server when the email is opened.
This gives the sending party the time the email was viewed, the IP address of the viewer, and application the viewer used to open the email. Often a unique identifier is appended to the request to allow the web server to relate the request to a specific email.