This is a slang term used in baseball to refer to no-hitter, a game in which the pitcher does not allow any hits. The term applies in all situations, where one is referring to a no-hitter, including before, during, and after a game. For example, a teammate may congratulate a pitcher by saying, "Great job pitching that no-no!" There is no special qualification for using the phrase "no-no," it applies equally to all no-hitter situations.
The slang comes from the first part of the phrase no-hitter, and has become more popular in modern times. The Major League Baseball record holder for most "no-no's" is Nolan Ryan, who pitched a total of seven no hitters. A no-hitter should not be confused with a perfect game, which requires that a pitcher not allow any hits, runs, or walks. Although still difficult to achieve, a no-no is easier to get than a perfect game.