Serve your clients before they leave! You have to earn a certain amount of points to pass to the next level. If you fail you lose a star, and if you remain without stars you lose the game.
When a client arrives, click and drag him to a seat; ideally you should match the seat and customer color (you can relocate them if necessary). After a customer orders, click the order and then click again to release it on the bar; when the dish is ready on the bar, click it and then click on the customer to deliver it. When he asks for the bill, click it to bring it to him, and when the table is empty, click it to pick up the coins and remove the dirty dishes.