Elephants do many different things with their big ears. One thing is that they can cool themselves down when they are hot. The blood flowing through the ears is close to the outside of the elephant body, and heat can be released as they flap them about. Some scientists also think that the big ears may funnel sound into their inner ear for better hearing. Elephants can also use their big ears to make threats. When they spread them out wide, elephants look even bigger than they already are, and they can use this posture to threaten other elephants or other animals.

source: https://sciencefriday.com/articles/why-are-elephant-ears-so-big-and-other-pachyderm-questions/