what does it mean in minecraft when you hear a scream?

Answer 1

It means that you have spawned Herobrine. Herobrine is Notch's so called dead brother.

Herobrine has white eyes and will try to kill you and if you are far away will just stare at you and disappear. Sometimes he spawns a lot of poop on you to try to kill you. Best to just not spawn him.(Maybe just stay on creative mode.)