A checklist is a way of keeping track of subtasks within a card. You can add multiple checklists to a single card.
Add Checklists via the "Add Checklist" button in the "Actions" section of the card detail. Enter a title (or use the default "Checklist"), then click 'Add'. Add new checklist items via the "Add checklist item" input in a checklist.
Mark an item as complete or incomplete by clicking the checkbox to the left of the title. Rename a checklist or checklist item by clicking the title, changing the text, and clicking save. You can reorder a checklist item by dragging.
Delete a checklist or checklist item by clicking the menu button that appears when hovering, and selecting delete.
Copy Checklist Items
Copy the items from another checklist on your board to a new checklist by selecting the checklist you wish to copy from the "Copy Items From..." drop down menu in "Add Checklist."
You can also turn a checklist item into a card by clicking the item and selecting "Convert to Card". It removes the item and creates a card with the title of the item beneath the current card.