Backmasking is the process of reversing an audio signal and placing it in something meant to be played forwards. When played normally the message will sound like gibberish, however, when the song is played in reverse the original message can be heard.
Some of the first instances of reversed audio were the result of The Beatles’ experimentation during the recording of Revolver. Since then, backmasked messages have turned up in all kinds of music with messages ranging from humorous to satanic.
Today, reversing audio is a popular way to censor explicit words for radio.