The preheader is the text, that follows the subject line and is displayed in the email client, even before the user has opened the email.
Usually, the preheader contains a summary of the email. The length of the preheader is limited. Different email clients and devices can show more or fewer preheader characters, but their number should not be too large.
A well-written preheader significantly increases the chances, that a user will open your email. Therefore, we highly recommend not leaving this field empty.