According to Hesiod, Pandora’s opening of the jar released evils into the human world.
In Works and Days, the harmful contents had previously been hidden from humanity. Once the container was opened, these evils spread among people, helping explain why sickness, hardship, and other forms of suffering exist. The episode belongs to Hesiod’s larger account of the deteriorating human condition.
The ancient poem does not present the contents as a neatly itemized list of named monsters. Later retellings often turn them into visual creatures or specific concepts, but the original account uses the broad idea of evils. That makes “evils” the most direct answer to what escaped.
Pandora’s action is also connected with the punishment of humanity after Prometheus obtained fire for people. The story therefore functions as an etiological myth: it gives a divine and legendary explanation for conditions familiar in everyday life. Modern uses of “opening Pandora’s box” retain the basic idea of unleashing many unforeseen troubles.