You can link cards to other cards to add structure to your card collections. For example: link task cards to a specific project cards. Or contact cards to an organisation card.
Before linking cards together you first need to add a relation to the card types you want to link.
Start by navigating to the card you want to link to another card type. In the example of linking tasks to a project navigate to the task card type
Add a new field to the task card, select "Link to card" as the field type
You can now select the project card type
Done, you can now save the card type.
You can now work with the relation in 2 ways:
Create a new child (i.e a "Task") card, here you can select to which project(s) the task should be linked
Open the details view of a parent (i.e a "Project") card by clicking the title of a card. You can now add tasks directly to a project.