To see the Opens counter for an email, from the main Email tab, select Track for an email.

The Unique Opens counter captures the number of unique email addresses (recipients) who click on the email to open it from their inbox or scroll through it without clicking on it such that the email shows within their preview pane. Each recipient is only counted once regardless of how many times they view the email.

This email statistic is prone to error because the preview pane in many email client programs will trigger a view regardless of whether the recipient actually read some or all of the email.  

Because this counter uses images to record opens, it does not count recipients that opened the email to view it who have images turned off or disabled in their email client.