Email to card allows you to add comments on cards from emails sent to a generated address. Once enabled in the organization, a new item will appear in every card's more menu.
Sending messages to a card's email address will add comments.
The subject line will be in bold and the body will be on a new line. The activity stream will also show you that the comment was created via email. As you can see at the bottom of this activity stream, the card itself was created via email, find out how here.
NOTE: Email to board/backlog and Email to card uses a third party service called Mailgun. Sending emails to Favro means that the data they contain will be handled by Mailgun.