Adding two identical signals results in a 6dB (6.02 actually) volume increase.
Now, we use two speakers. Left and right.
But … we mostly “pan” signals to the center. Where there is no speaker.
This is called the “ghost center”.
Now, if you pan a mono signal to the left speaker only, you “hear” (so to speak) a level of 0dB
Same for panning a signal full to the right channel.
However, when panning the signal to the center, they get added, resulting in a 6.02dB volume increase.
Which means that -for keeping the same balance- you would have to lower your mono-fader by 6dB to “fit” in the mix in the exact same way as when panned hard left or right.
To compensate for this, there is pan-law.
When you set the pan law to -6dB, the theory says that when panning a signal from left to right, you wouldn’t have to compensate the level with the fader, but being compensated by the pan-law.
In practice, this is slightly different; and it depends on the position of your monitors.
The best way to figure it out is panning a mono track from left, over center to the right.
With the correct pan-law, you shouldn’t hear a drop (or increase) in volume while panning.