Headroom
The gap between the level you normally work at and the level where the signal clips.
Analogue gear has a nominal operating level with twenty-odd decibels above it before distortion turns audible. Digital has a hard ceiling: above 0 dBFS there are no values, only flattened tops. So you record with margin, and EBU R 68 bridges the two worlds by aligning 0 dBu with −18 dBFS.
The advice to "record as hot as possible" dates from the 16-bit era, when every decibel of headroom cost you noise floor. At 24-bit bit depth there is over 140 dB of theoretical range and headroom is practically free. Mastering then eats it all back up, which is more or less what the loudness war was about.