Task Dependencies allow you to create relationships between cards in order to set priorities for your team.
Thanks to this feature, your team members will know instantly which card they should start working on as soon as they’re done with their current one.

To set a dependency between two cards, simply hover over one of them in your Gantt chart, click on one of the anchors that appear on both sides and drag towards the second card’s anchor.

Moving either one of the tasks linked by a dependency relationship will affect the other. So the tasks move in consequently.

WARNING
If you duplicated a Trello Board, dependencies won't duplicated in this new Trello Board.