Combo's are a way in the game to make additional coins and are part of daily challenges and achievements.

You can earn combo's by making sure that you serve full orders and by making sure you serve a next order before the combo timer runs out:

Whenever you serve a full order before the timer runs out, the combo counter (x1 on this one) is increased by 1. The higher the better., especially for levels where you need to make a certain amount of coins. 😊