You can add a preheader to your newsletter while creating a template. A preheader is a short text that follows the subject line when an email is viewed in the inbox. It can be a short summary of the email content.
To add a preheader, click on the "Preheader" button in the upper right menu bar.
A new window appears, where you can write the text you wish for the preheader. You can also see example images, how the preheader is displayed in common email clients.
If you don't add a preheader to your template, the email clients automatically show the first text found in the email campaign.
Where the preheader is located in Outlook:
Where the preheader is located in Gmail: