You will see these metrics reported in many of the reports offered in the analytics dashboard:
Unique open is a metric that includes the first open response per recipient. One open is counted per recipient of the email, even if they opened the email more than one time.
Total open is a metric that includes all open responses for all recipients. All open activities will be counted, including multiple opens for the same recipients.
If I receive your email and open it 5 times, my email address would be included in the unique open count one time and the total open count 5 times.