It's a classic restaurant serving game: when a passenger orders a product you have a limited time to deliver it. Don't deliver the wrong product or they will be upset!
After a customer orders, you first click the product he asked for, then the passenger that will receive it. You can take up to 6 items at the same time (you see them at the right bar) to avoid having to come and go for each order. If by mistake you take the wrong item, click again its image to cancel it.