1st Open email alerts notify you by email when a tracked email has been opened for the first time by each recipient. For example, if you have sent the email to one recipient you will receive one alert, but if you sent the email to 2 recipients you will receive 2 email alerts.
This feature can be enabled/disabled in your Mailtrack settings...
...or by clicking on the links at the bottom of an email alert you've received.
Please note that, for technical reasons, if you disable email alerts you may still receive email notifications for tracked emails that you sent prior to disabling the feature.