What is Mastering?
Ultimately in mastering you're trying to make decisions about whether to change something.
Mastering is zooming out, recognising the forest instead of the trees. It's making the final changes to a track, ensuring that it's ready to go out into the world and that the listeners will have the experience you want them to have.
Level vs Loudness
The level is how high the peak or RMS values are on a meter. Loudness is entirely in your head. Loudness happens in your ears and your brain, not in your D.A.W.
Loudness Potential = a recording with a strong mono-component, compared to the stereo information. Another way of phrasing it could be, mid vs side, or uncorrected vs correlated signal. These elements will change the loudness potential of your track. A mono recording will sound louder than a defuse recording that's primarily out of phase.
We need to understand the tone of the material we're working on and the mono vs stereo-width to understand the loudness potential.
Are There Any Rules in Mastering?
Mastering is a game of inches and gentle nudges. 0.5dB there, 1dB here. 2dB is considered a lot in mastering.