This refers to MPEG audio layer3. It is used to compress audio signals. It compresses audio by using following techniques. MP3 divides the audio frequency spectrum in to 576 to different frequency bands. Then it compress each band independently using three coding schemes namely layer1, layer2 and layer3. Normally the human ear is good at hearing mid range frequency sounds. So mp3 records only the mid range frequencies and drops all other ranges. So the audio sound is compressed. If a sound frequency is identical in different channels, it stores only one copy.