A preheader is the text following the subject line when an email is previewed in desktop and mobile email clients. The preheader is an optional addition to any campaign. Since the preheader is one of the first three items seen when viewing an email this text is typically a call to action or other catchy text to call attention to your email message.
You can set up the pre-header in Step 1 (Setup) of the Create a Campaign Workflow. It is possible to set this information when building the template but it is recommended to wait until creating the specific campaign to remove the possibility of mistakes.