The fourth wall is a theatre term referring to the invisible wall between the stage and the audience. "Breaking the fourth wall" occurs when the cast talks directly to the audience or otherwise acknowledges their existence.
By extension, "breaking the fourth wall" in a work of interactive fiction is said to occur when, within the game world, the work speaks directly to the player (as opposed to the player character). The fourth wall can also be broken indirectly when the PC is told s/he is in a game, or that the world is fictional, or that the something will help them earn a last lousy point, or told any other meta-information about the game itself.
Usually meta-information about the game is presented by responses to meta-commands such as ABOUT, HELP, or HINTS, or by the "narrator" (a.k.a. the voice of the parser) via asides like "[I don't understand the word 'scratch'.]" Such game responses are not considered part of the game world, and thus are not considered "breaking the fourth wall".
Compare and contrast with the concept of breaking mimesis.