What is the difference between 0 dB and 0 dBFS?
We talk about differences in sound levels in decibels.
Differences in sound level.
So one sound can be 10 decibels louder than another. Or you can take a sound signal and make it 10 decibels quieter. Or you can leave your fader at 0 decibels and make absolutely no change at all.
So that is the meaning of 0 dB - no change in level.
You cannot say that a sound has a level of 100 dB. You cannot do that.
When you see something like, "The noise coming from the freeway is 100 dB", that is absolutely meaningless and written by someone who has no knowledge of decibels. (They probably mean 100 dB SPL, but that's another topic.)
0 dBFS on the other hand refers to a specific level.
'FS' stands for 'full scale'. 0 dBFS is the level of a signal that is at the maximum level your system can cope with without clipping the tips of the waveform.
So -10 dBFS means a level that is 10 decibels lower than the maximum level your system can handle.
In summary - decibels are used to describe differences or changes in level. 0 dB means 'no change'.
Values in dBFS are used to describe signal levels in comparison with the highest level your system can handle.