dBFS stands for decibels relative to full scale, a unit referenced to the maximum level digital audio can represent. A typical peak meter puts the ceiling at 0 dBFS and shows how far below it the signal sits as a negative value such as -6 dBFS or -12 dBFS.
0 dBFS is the ceiling
In fixed-point digital audio, peaks above 0 dBFS cannot be written to the file correctly. Exceeding the ceiling causes clipping, so when recording and exporting, keep peaks off 0 and leave some room.
Negative numbers still make sound
-12 dBFS is not silence; it means 12 dB below full scale. The closer the number is to 0, the higher the signal level. As the signal approaches silence the value gets smaller, and meters may display it as minus infinity.
Difference from dB and sound pressure level
dBFS is a value relative to the digital ceiling and does not directly express how loud the speakers actually are. Two files at the same dBFS can produce different sound pressure in the room depending on the audio interface, amplifier, speakers, and volume setting.
Floating point inside the DAW
In a DAW using floating-point processing, intermediate signals above the equivalent of 0 dBFS are not necessarily clipped immediately. The final output and fixed-point files still have a ceiling, though, so the fact that levels can go over internally does not mean headroom is unnecessary.
Difference from LUFS
Peak values in dBFS are good for managing the instantaneous maximum level, but they do not tell you how loud the whole song feels. Use LUFS for perceived loudness and True Peak for the maximum level including between samples, and read the meter that fits your purpose.
