A term used in drama and theater, an aside happens when a character's dialogue is spoken but not heard by the other actors on the stage. Asides are useful for giving the audience special information about the other characters onstage or the action of the plot.
Shakespeare, everyone's favorite drama dude, uses tons of asides. In Act I, Scene ii of Hamlet, Hamlet, in full snark mode, says to the audience about his uncle, Claudius, "A little more than kin, and less than kind." He's referring to the fact that his dear uncle has, presumably, killed Hamlet's dad, and then taken up with the guy's wife. But of course he can't say this to Claudius's face. Awkward. So he says it to the audience instead.