UPDATE: What is a tester?

Testers are members who are capable of receiving any test messages that you may want to send before launching an actual mailing. 

Create new or delete existing testers from the Testers page in the Campaigns menu. Testers are created on the testers screen of the Settings section. 

Note: Every tester must be a member of the brand. This is because many mailings contain personalized information, such as a greeting which incorporates the member's first name (e.g. "Dear ##FirstName##,") or store personalization like ##Store.StoreAddress##.