Using static and dynamic placeholders in your mailing templates can make it hard to imagine what your sent mails will look like eventually. With the preview function you can check the final result of your mailing template.
go to your campaign
open the template you want to preview
click on Preview changes in the bottom right of the screen.
In order for the preview to work, you need to make sure that you already added prospects to the campaign and that the formulas used in your campaign are correct.
Furthermore you can also send yourself a test mail, by clicking Send a test email in the upper right corner of the window.