The "Plain Text" version of an email was originally designed for older email clients that couldn't display HTML layouts. This allowed users with older machines or software to read emails whose HTML could not be displayed. These days, now that most email clients are able to display HTML, plain text "backup" content is rarely shown to users and is mostly used to help mailbox providers read and score the content of an email. Within the email editor, you have the ability to review and edit the plain text version of an email, if you wish.
Within the email editor ( either HTML or Drag and Drop), look for the dropdown on the upper right of the screen, and click Plain Text.
Any text over from the HTML or Drag and Drop version of your email will be copied into the plain text version. It will be stripped of any code, like HTML or merge tags. Review and edit your plain text email to make sure everything is how you want it. Then click Save text.
When your email is sent, the plain text version of your email will display to any users who are unable to view an HTML version of email. This content is also scanned by mailbox providers and is used to help sort messages within the inbox.
To view what the plain text version of your email will look like after it's sent, send a test version of the email to yourself. Pull up the email in your inbox, and look for "View source", "Show original", or "View raw message", depending on the email client you are using.