Wikipedia has a reasonably easy-to-understand description of Replay Gain.
I have tagged all my 10,000+ tracks with album and track Replay Gain values. With my SqueezeBox I enable "Smart Gain" for playback. This means that when I play a mix of tracks from various albums there are no wild swings in loudness between tracks because it uses the track Replay Gain value to normalize the volume. If I play an album, it uses the album Replay Gain value, which is the same for all tracks on the album, so the tracks sound correct in volume relative to each other as the mastering engineer intended.
