Also called “backward masking,” this is the technique of recording a sound or message backward on a track normally meant to be played forward. The Beatles were one of the first groups to experiment with this. There are a variety of things on their later albums which, when played forward, sound like part of the music, but played backward they become a recognizable sound or vocalization.
Often times the sounds that are heard, even when the recording is played backward, 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 messages in recordings in this fashion, but little of it has ever been conclusively proven.