1) An inquisitive remark; a way of asking "are you for certain that what you are expressing right now is something that can be trusted?"
2) A confirmation of seriousness.
3) A deep and philosophical question, as in: "Are you sure that you are not dicking around right now?"
4) An exclamation.
1) Are you for cereal?
2) For cereal.
3) SUPER cereal?