Some levels have tasks that you have to perform to pass the level. You can see the task at your left hand  when you enter the level. It gets a green check mark when the task is done. You still have to reach a certain amount of points as well to pass the level. An example of a level with a task is level 39, that has to be cleared out within a short period of time.

At your left hand you can see the task

You know that  the task is completed when  a green check mark is shown.