An email campaign is a set of emails that can be sent to an email list (or a segment of a list).
The Campaigns index page summarizes some information about your campaigns. You can see the status of the campaign (editing, scheduled, sending or sent), the number of people it was sent to, the number of times it was opened and how many links were clicked, and when it was or will be sent.
The percentage below the opens and clicks is as part of the total amount of subscribers that received your email.
Are you a visual learner?
This video shows you a general introduction to using campaigns.