Open rate tracking lets you see if your subscribers have opened your emails from their inbox. Open rate is mostly influenced by the attractiveness of your subject line.
After enabling the tracker and sending a mail merge, you will notice in your spreadsheet that the status will change from 'EMAIL_SENT' to 'EMAIL_OPENED' for each email opened (in column after the 'Merge status' column).
A note in the merge status cell will also indicate when the email has been opened, plus some additional info about WHO (email address), WHERE (ip address), DEVICE (email client, OS):
Same info you can find if you visit Your MailMerge Dashboard:
MailMerge embeds a tiny, invisible tracking image (a single-pixel gif, sometimes called a web beacon) within the content of each message.
When the recipient opens the message, the tracking image is scanned, referenced and recorded in our system.