This is slang from the UK although it is used in Australia as well and can be used in number ways:
1 To make a foolish statement;
2 To talk nonsense or rubbish;
3 To say something which is quite obviously false;
4 To exaggerate your abilities or knowledge of a subject;
5 To bluff or boast about something;
6 To be a self-aggrandising twat;
A term to indicate someone's questioning of the validity of one aspect in a fictional story is in fact overshadowed by the absurdity of the story's basic premise.
A phrase first coined when one person loudly wondered how the ducks in a Donald Duck story managed to breathe under water without any visible breathing apparatus on their diving suit, and another replied "You know you're reading a story about talking ducks, right?"
An expression used to diffuse responsibility for an unpopular statement made in a public setting. Typically used to imply complicity or collusion on the part of an unwilling stranger.
You: I mean, really, who hasn't made out with a rundown fat chick in a moment of drunken desperation.
Crowd: *silence*
You: *smile and point to a random guy in the crowd* This guy knows what I'm talking about.