Loudness is an important issue in the mastering process. Since loudness and dynamics in mastering get in the way, a little sensitivity is needed here.
The volume of a recording is measured in decibels. A distinction is made between the measured volume and the perceived volume (loudness). The latter is given in dB / RMS. Basically, one can say: the fuller the appearance of the level in the sound program looks like, the higher the dB / RMS.
So you can normalize a song to 0 dB and still it sounds quieter than another song, also maximized to 0 dB. Then both songs measured are equally loud, but the latter has the higher dB / RMS value.
Distance between loud and quiet
This results from raising the softer segments of the song and reducing the gap between loud and quiet. The dynamics are thus reduced. The more offensive you do this with a limiter, the louder the song and the lower the dynamics.
In recent years, this has been heavily exaggerated. But now I find more and more often that musicians again put more emphasis on a higher dynamics. With my mastering, I try to achieve a high loudness, but still maintain a necessary level of dynamic.