Each time you win a battle, an incubator reward is dropped into one of the four available reward slots. In order to retrieve the resources from your incubator reward, you can activate its timer and wait for it to finish counting down, or you can pay Cash to unlock it. Once an incubator is opened, the slot it took becomes vacant. 

If all four battle slots are occupied, and you win another battle, you will not receive an incubator reward. In order to receive an incubator for winning a battle, you must have at least one vacant reward slot.