What does pandora box mean

It's a term from a story in Greek mythology. From Wikipedia (underlining is mine):

After Prometheus' theft of the secret of fire, Zeus ordered Hephaestus to create the woman Pandora as part of the punishment for mankind. Pandora was given many seductive gifts from Aphrodite, Hermes, Charites, and Horae (according to Works and Days). For fear of additional reprisals, Prometheus warned his brother Epimetheus not to accept any gifts from Zeus, but Epimetheus did not listen, and married Pandora.

Pandora had been given a large jar and instructed by Zeus to keep it closed, but she had also been given the gift of curiosity, and ultimately opened it.

When she opened it, all of the evils of mankind escaped from the jar, although Pandora was quick enough to close it again and keep one value inside--Hope.

people will say " your opening a whole pandoras box here" meaning a whole load of trouble.Putting the cat amongst the pigeons or opening a hornets nest, all basically mean the same.

It's just a figure of speech which means... sometimes things are best left alone