If you have a card that requires a large amount of effort, you may want to break it down to a backlog or a board.
If you break down the card to a board and a backlog, your card will reflect this as seen below.
These notes on the card are called relations. On your lists and boards, you will find an icon next to the context menu signifying it has been broken down from a card.
If you click this icon, the card it is broken down from will pop up and you can edit the parent card from here.