To learn more about this, we highly recommend Litmus' informative article. They start by giving a great explanation of preview text:
"Preview text is a snippet of copy pulled in from the body of your email and typically displayed underneath the from name and subject line in a subscriber’s inbox. It is alternatively referred to as snippet text or a preheader."
We recommend using their solution under the heading "Hidden preview text". They describe hidden preview text as:
"...allow[ing] you to customize the text that is displayed in the inbox without displaying that text in the body of the email."
Their recommend solution involves using CSS that instructs the text not to display, and we have found this solution to work well:
Insert preview text here.
For more, please see the full article.