The Open Tracking app adds an invisible image at the end of the email which can track e-mail opens. If the email recipient has images enabled on the email client, a request to server for the invisible image is executed and an open event is logged. These Events are logged in your Statistics portal, the Email Activity interface, and are reported by the Event Webhook.
When using this service customers often ask the difference between a unique open versus an open. A unique open is logged only the first time a given recipient opens the email whereas normal opens are logged for all opens of the email in question.