Minecraft- How does Herobrine work?

My little brother LOVES mine craft. Recently, he updated mine craft on his iPod, so now he can do more things. Earlier today, a server came up called Herobrine and he entered. Herobrine was there walking,building etc. My brother was able to have a conversation with him and Herobrine knew where he was.
Who s behind Herobrine?
How does it work because I thought he was just like a mine craft pig! You can t communicate with them!
Is someone talking to my brother over a screen. And if so is it something I should be worried about? How did he find my brother?
Thanks!

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