Mastering is in fact leveling, simple as that. Of course, there are numerous ways how to reach these levels and it does not only involve pushing the “Master” fader on the mixing console up or down.
Further, mastering does not mean one can only attach a limiter to a master buss and expect that pushing the fader up or down defines the process as “mastering”. In fact, mastering is a complex set of steps and techniques for which individual engineers learn separately. For that purpose, we have “mastering engineers” who specialize solely in mastering. Before engaging into mastering, the song needs to be properly mixed. Many people I know who work in music production, do not even separate the process of mixing from mastering. In fact, they begin mastering while they mix and “if something sounds wrong, then they come back and push a few knobs here and there until the mix comes to terms so the master buss can be accordingly adjusted”. This procedure is very wrong and is not how mastering works indeed.
One prominently adverse phenomenon that plagues the mastering world is the “ear saturation” or rather a bias that we experience when exposed to listening of certain tracks for longer periods of time. In other words, we become tired subconsciously and our ears adapt to the sound that we listen. That is why, a certain mix can sound very differently when listened to in the early morning, compared to how it sounded the night before. For this reason, mixing engineers avoid to master music they have mixed or avoid doing it the same day or even one day afterwards. One practical way to remedy this inadequacy, is to schedule the mastering sessions of diverse tracks the following day from the ones that are being mixed, and vice versa. I personally like to take breaks and rest a day before making any mastering attempts. Another trick I use is to listen to the mix on different speakers, even headphones and try to identify potential problems before attempting mastering to eliminate any bias that can disrupt the process.
Assuming a well-balanced mix, the mastering engineer will begin analyzing the tonal balance of the mix before applying any sort of plugin or hardware to the master buss. In reality, the idea of mastering process is to deliver the end product to levels appropriate for publishing to certain platforms (i.e., YouTube, Spotify, and so on), with minimal to no change of the way the mix sounds. This means that during mastering, we avoid to apply any equalizers, saturation, compressors or other tools which may impact the way the mix sounds. There are of course brighter, muddier or punchier masters because of this reason, but this highly depends on the genre and the preference of the clients. That’s why, the clients usually select one of multiple master versions with which they are satisfied how it sounds and the rest of the tracks or album is built upon this example. There are instances when the mastering is being done with only a master buss processing unit or a plugin, like a maximizer, limiter or analog hardware, but in this case usually the mix is solid and well balanced and does not require any interventions and the plugins and hardware will respond properly. Thus, the master can be pushed to higher levels if needed.