Once you've created a campaign, you can test it out before you send it to everybody in the corresponding mailing list. To do so, click thebutton.
You will then be prompted to enter an e-mail address. This is where the test e-mail will be sent. In order to simulate a real e-mail, the details of a random user are included in the test e-mail.
You can only test a campaign while it is in draft mode.