When Connect sends out an email it embeds a 1x1 pixel at the bottom of each email. When a customer receives this email and has enabled images to display then Connect marks this email as opened.
If a user has disabled image downloads in their email client they can still read the email and Connect wouldn't know about it, but the customer can still click links in the email. In this scenario the customer would appear to have not read the email but they did click on a link.
Opens/Reads only works for HTML based emails and is not available in TEXT, SMS, etc