Stunned is a condition added in the Second Edition base game.
Condition text: 
: Discard this card or token. This is the only action you may perform on your turn while you have this card or token.
Creatures who are stunned must use an action to discard the stunned condition. That is the only action a stunned creature may perform as long as it is stunned. However, after using an action to discard the condition, both heroes and monsters can then use his or her second action per standard rules.
Back side.
Front side.