ReplayGain is used to calculate the loudness of the audio file. ReplayGain values (track peak, track gain) are written to the metadata of the audio file.
ReplayGain calculation does not change the audio, but the loudness can be adjusted in a ReplayGain capable player during the playback so that audio files from different source will play at the same volume level.
Easy CD-DA Extractor uses ITU-R BS.1770 loudness calculation algorithm for most accurate calculation of loudness.
More information about the ReplayGain: http://replaygain.hydrogenaudio.org and http://en.wikipedia.org/wiki/ReplayGain