Refers to the technique of putting material in recordings that can (usually) only be heard or deciphered when the recording is played backwards. The Beatles were one of the first groups to experiment heavily with this. There are a variety of things on some of their albums which, when played forward just sound like part of the music, but played backwards become a recognizable sound or vocalization.
Often times the sounds that are heard, even when the recording is played backwards, are so difficult to recognize that it is debatable whether they are just a coincidence or are an intentionally placed message of some sort. A number of artists have been accused of putting hidden satanic messages in recordings in this fashion, but little of it has ever been conclusively proven.