Pandora's box refers either to the ancient Greek myth of the first woman, Pandora, who opened a box that released all troubles into the world, or to a modern to a situation where a simple act unleashes more troubles or problems than they anticipated. The modern allusion derives from the Greek myth.
The seventh century B.C. poet Hesiod first tells the myth of Pandora as the first woman, created by the god Hephaestus at the command of Zeus as a gift for the Titan Epimetheus. Pandora was so beautiful the gods showered her with gifts. Zeus, out of jealousy, gave her a sealed jar, and told her not to open it. Later generations mistranslated the word for jar as box. Pandora, being unable to control her curiosity, opened the jar, which was filled with all the troubles of the world, such as illness, pain and hunger. All of these troubles escaped, and can never be sealed back into the jar. So in Greek mythology, Pandora's box was a story of how all troubles came into the world, and closely mimics the Judeo-Christian story of Eve. As with the story of Eve, many modern readers believe the story reflects a sexist attitude toward women.
The modern meaning of a Pandora's box is a simple act that results or could result in unanticipated problems that cannot easily be overcome. It can refer to an act that has already been performed, or it can be a warning from someone who anticipates problems if an act is performed.