Not exactly helpful, but here's what the AIM says about them:
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Advisory Circulars - The FAA issues Advisory Circulars (ACs) to inform the aviation public in a systematic way of nonregulatory material. Unless incorporated into a regulation by reference, the contents of an advisory circular are not binding on the public. Advisory Circulars are issued in a numbered subject system corresponding to the subject areas of the Code of Federal Regulations.
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That's kind of a regulese way of saying that they are publications put out by the FAA that discuss aviation topics but without going thorugh the formality of the proposal-comment-publication process that regulations go through. The comment about not being binding on the public is a bit of an overstatement: it's technically correct but, like the AIM itself, may end up being used to set a standard of "reasonable" conduct.