Preheader text is brief summary text that follows the subject line when an email is viewed in an inbox. Many email clients provide them to give you a preview of what the email contains before it’s opened. Here’s an example in Gmail:
Unless you provide specific preheader code this text is taken from the text in the email body. You can override this with a small summary text to make the email more engaging and likely to be opened with the preheader code below by placing it just after the <body> of the email template or in a text block at the start of your email.