To stun someone is to hurt them or shock them so badly that they can't think or move.
Something has stunned this bear! Maybe he fell down so hard that he can't move. Maybe someone hit him very hard, and now he's weak and dizzy. Or maybe something shocked or surprised him so badly that he can't think straight.
Someone has stunned this dinosaur! Maybe his wife just told him, "Guess what? We're having seven babies!" He's stunned, so he's just standing there, eyes wide, mouth open, too stunned to realize that he's dropping his drink.
When something stuns you, you stand still, you can't move, and you can't speak. Usually that's because you've gotten bad news, or shocking news. But it could be because you've gotten some very surprising good news!