A new combo goal has been added in season 4, episode 5. This means that, for levels with a combo goal, the goal is to earn a certain amount of combos (rather than serving a certain amount of dishes).

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:

Check out this FAQ article also for tips! https://gamehouse.helpshift.com/a/delicious-world/?p=web&s=gameplay&f=how-does-a-combo-work