Opens are currently tracked via a tracking pixel included when that contact opens your email. When that image is downloaded from our mail server (Mailgun) that constitutes an 'open.'
The open rates are reported are accurate but the number can be inflated. Some recipients may have a preview option allowed via their email, that can trigger when that email is clicked or scrolled to view in that preview mode. This act of previewing that message, dependent on their email settings, can also cause that open rate to count.
Additionally, if the contact were to forward that email, future forwards may also constitute a separate open count.