Lossy compression is a form of compression that significantly reduce multimedia file size by throwing away information imperceptible to humans.
Human audio perception is not perfect. Lossy compression takes advantage of thse characteristics. By selective discarding, a great deal of unnecessary information is thrown away. The amount of information discarded is usually adjustable, giving a compromise between 'small size bad quality' and 'high quality large size'.
The downside to this is that the waveform reconstructed from compressed information will never exactly match the original waveform.