The "unique" count refers to the number of matching prospects that performed a particular action, while the "total" count represents the full amount of times that an action was completed.
For example, the screenshot above shows that 371 unique prospects opened the email. Since the email was opened by some prospects more than once, the total opens is 792.
In terms of visits, the email generated visits by 17 unique prospects, and these prospects visited a total of 72 times combined.
Since it is entirely possible for prospects to do something more than once, you may find that the "total" stat will often be higher than the "unique" stat.