Headroom
Headroom is the difference between the peak level of an audio signal and the maximum digital ceiling (0 dBFS). Leaving sufficient headroom—typically -6 to -3 dBFS on the master bus—allows transients to pass cleanly and gives mastering engineers room to process without clipping.
In digital audio, 0 dBFS is the absolute maximum. Any signal that exceeds it clips hard, creating harsh distortion. Headroom is the safety margin you maintain below that ceiling throughout your mix.
For a typical EDM mix, a common target is to keep the master bus peak between -6 and -3 dBFS before mastering. This gives a mastering engineer (or mastering plugin) enough room to apply limiting and loudness maximization without destroying dynamics.
Headroom also affects how transient sounds like kicks and snares behave. A mix with 6 dB of headroom lets the kick punch through cleanly on every hit. A mix that's already at -1 dBFS leaves no room for transient peaks and will sound squashed before any mastering is applied.