Autopilot uses a tracking pixel to detect when a recipient opens an email. It is a tiny, invisible image downloaded when the email is opened. This is the industry standard for open tracking.
Most modern email services have images enabled by default, however sometimes a recipient may turn off images. To counter this, Autopilot will also record an open event if a link in an email is clicked or replied to.(since to do either of things, the contact must have opened the email).