When a reply is received to an email you've sent with the Email for Trello Power-Up, a comment is added to the card containing the content of the email. And, a Trello notification icon is displayed on the card. Once you open the card, the notification disappears.
You might want to return to the card to read or take action at a later stage, but with many cards on a board, it can be difficult to find and keep track of the card you want to return to.
The "Mark Unread" button allows Email for Trello users to flag a card notification as unread. Simply click on the "Mark Unread" button on the card to set the notification as unread. The Trello bell icon will show on the card and remain until you open and view the card.
For tips on setting up additional notifications, visit this article: Notify your team when you get an email reply in Trello.