This app records audio, and embeds a secret text message into the audio file. You can then share the recording with others, who can extract the secret message with the same app. The messages can be encrypted with a password for extra security.
The app uses a process called Echo Steganography to embed the secret messages. The difference between an unencoded and encoded audio file is inaudible to the human ear, only complex steganalysis can determine whether the file has any data hidden in it. This method of data hiding is also resistant to transcoding between audio formats.