When looking at your program reports, you will notice that your Opens and Clicks tracking stats are broken into two categories: Total Opens/Clicks and Unique Opens/Clicks.
Total:
Total opens and clicks shows the total number, meaning if a target opens an email or clicks on a link twice, it will be counted twice.
Unique:
Unique opens and clicks shows the number of targets who opened an email or clicked a link. If a target opened an email twice, it will only be counted once in unique opens. Unique stats are more useful for measuring the success of your programs because you might have targets who open an email multiple times, ultimately influencing your opens rate.