Breaking the fourth wall refers to a situation in which a character reveals his or her awareness of the audience (this can also be called metatheatre) or the fact that he or she is in a fictional story. The technique has been used for millennia: it was standard practice in Greek comedy. For instance, at one point in the Greek playwright Aristophanes' play Peace, the hero Trygaeus (who is being lifted into the air by a crane situated offstage) tells the crane-handler to be more careful.
In reference to the Smurfs cartoon show, there have been some instances where a Smurf character "breaks the fourth wall". At least in a few known instances, Papa Smurf has done so at the end of "The Fountain Of Smurf", "All Creatures Great And Smurf", "Papa's Day Off", and "Smurfing Out Of Time". Grandpa Smurf has done this in "Lost Smurf" during an escape, Grouchus Smurfus has done this in "The Monumental Grouch", and Vanity Smurf has done so at the end of "A Loss Of Smurf".
In the 2011 Smurfs movie, Narrator Smurf does this as he provides narration for the beginning of the movie's story. During the end credits when they show Gargamel and Azrael still in New York City, Gargamel looks over to the camera saying, "What are you looking at?", before zapping the camera with what's left of his wand.
There's also a bit of fourth-wall breaking in the Smurf comic books, usually at the end of the story.