What’s headroom, you ask? Headroom is the space between the loudest peak in your mix and the maximum the recording could be without clipping or distorting.
To maximize the mastering process, you’ll want to leave plenty of space for the compression and limiting processes to go to work optimizing the loudness of your mix. When done properly, your track will end up with a much wider dynamic range, giving your tracks a wider and more open sound that make the loud parts hit harder, and the softer parts stay soft.
To get the best results, you should target your loudest peaks in your master stereo fader to stay between -3 and -6dB.