Pandora's box refers to an ancient Greek myth about a girl called Pandora. She was created by the Greek God Hephaistos and sent to earth by Zeus. However, she was also given a box filled with 'bad luck'. Pandora didn't know what was in it, and she was so curious that she opened the box. When she opened it all the bad things came into being: diseases, disasters and all kinds of trouble.
So when you're talking about Pandora's Box, you're talking about something that might bring bad luck onto you.