Mago has several features that utilize "open tracking" as a convenience for you to help you understand what is happening with your emails after you hit send.
For example, if you look in a record's history, there is an indicator on the left that indicates whether the email has been opened or not. Red means it hasn't been opened, and green means it has been opened at least once:
This can be a handy tool for you to quickly assess whether or not you're reaching your intended audience, but it does have some limitations and will never be 100% accurate.
For privacy reasons, some email servers are programmed to block open tracking. All methods of open tracking can be blocked if the recipient server doesn't want to allow it. When that happens, your emails to those contacts will appear to have been unread even when they have been. You will also not receive notifications that emails were read by these individuals.