Column view is is another way to visualize your project. Each column either represents an individual card, or stack of cards. In the case of stacked cards, the stack title will be displayed as the column header.
Another way to think about columns is: Column header = Stack name. If your card isn't in a stack, there's nothing to (re)name.
You can drag one card under another to create a new stack. To remove a card from being in any stack, tap the Edit button. Then select the card and tap Unstack.