What does it mean to open Pandora's Box?

It's a phrase that refers to the Greek myth about Pandora receiving a box from Hermes that she wasn't allowed to open, but she was so curious that she did open it. When she opened it all the bad things that are in the world today, like sickness, hunger, jealousy, anger or greed, flew out of the box. But hope was hidden in the box, and she was able to keep it inside. So when someone says that Pandora's box has been opened, or the like, they mean that bad things are going to happen.

It means you've revealed a secret or done something you shouldn't do and in doing so "great evils" or unforeseen negative consequences are released.