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.

The final result is that the information the beacon alone gives the sender enough information to deduce precisely when you opened the email.  You location can be derived to varying degrees of accuracy.  Your client, such as a particular desktop application, or model of phone is available.

When a unique identifier is included, the email address originally used is available.  This can then be combined with what is already known about you.  You are no longer just John Doe living at 123 Fourth St, but you also opened a particular list of emails at particular locations, at particular times, using particular equipment.