Once you've created a card and shared it with other people, they will likely comment on it to suggest changes or ask questions.
Share your "My Test Card" card with a friend now, and ask them to comment on it. Then click "reply" on their comment (you may need to reload the page) to write a reply.
When they write their comment you will receive an email letting you know, and the comment will also show up in the Updates Inbox.
That's the end of the Tutorial. Go forth and build some awesome cards!