normalising is the process of amplitude manipulation to make the loudest peak in the music as close to the clipping threshold as possible thus maximising the dynamic range of the playback medium. It does not affect the dynamic range or frequency spectrum of the material, that is compression. You can make the peaks of several tracks the same, or, measure the average subjective loudness across several tracks, and adjust the peaks to give the same average loudness, even though the peaks may differ, to avoid marked volume changes through an album, keeping all the time within the clipping threshold.

Most programs will normalise by converting to wave, adding the relevant numbers, and then converting back to mp3, something to be avoided since compressing and already compressed original again, thus losing more information. Mp3 gain is far superior because the normalisation is performed directly on the data frame blocks in mp3 format. Thus the volume adjustment is lossless and can be reversed to give a file identical to the original.