when you place your order, the cashier marks a coffee cup with your order and places it into a queue. This queue is literally a line of coffee cups on top of the espresso machine. The queue decouples the cashier and barista, letting the cashier continue to take orders even when the barista is backed up. It also allows baristas to start servicing the queue if the store gets busy, without impacting the cashier.
(Full Story: Starbucks Doesn’t Use Two-phase commits)