dBFS
Decibels relative to full scale — the measure of digital audio level, where the ceiling is zero and everything else is negative.
A digital system has a largest number, and 0 dBFS is that number. Everything below is expressed in negative decibels: −6 dBFS is half the amplitude, −96 dBFS roughly the floor of 16-bit audio. There is nothing above zero — push past it and the waveform is clipped flat, and digital clipping sounds considerably nastier than analogue tape saturating gently.
A peak meter reading −0.1 still doesn't mean you're safe. Between samples, the reconstructed analogue wave can overshoot zero — so-called inter-sample peaks — which is why streaming services ask for a true-peak ceiling around −1 dBTP. Calibration to analogue levels differs too: the EBU puts the studio reference level at −18 dBFS, SMPTE at −20. Two studios can mean different things by "normal level" and both be right.