What is the difference between unique opens and total opens?

What is the difference between unique opens and total opens?

Unique Opens - Number of recipients who opened your email campaign.

Total Opens - Number of times the recipients opened your email campaign.

For example:

Number of times Rob opened the email - 4
Number of times Doug opened the email - 3
Number of times Nick opened the email - 2

Total Opens - 9
Unique Opens - 3