“Open Pandora’s box” means to start an action that creates many unexpected problems.
The idiom comes from the myth in which Pandora opens a sealed container and releases troubles into the human world. In contemporary English, the expression is usually used figuratively for an apparently small decision with broad and damaging consequences.
The wording is a later development rather than the exact language of Hesiod’s Greek poem. Hesiod describes a jar, not a box, and the familiar English phrase gained force through later translation and retelling.
The idiom is related in meaning to “open a can of worms,” but the expressions are not identical. “Open Pandora’s box” usually emphasizes consequences that are difficult or impossible to reverse, while “open a can of worms” stresses the creation of a complicated problem or dispute.