Gain staging
Keeping the right level at every point in the signal chain, so nothing sits near the noise floor and nothing hits the ceiling.
In an analog chain every stage — preamp, EQ, compressor, summing bus — has its own noise and its own clipping point. Too low early on and the noise gets amplified later; too hot and some stage nobody is watching starts to distort. The aim is for each stage to run in its sweet spot with headroom to spare above. That is why a channel strip starts with a gain knob, not with the fader.
In a DAW running 32-bit floating point, internal summing practically cannot clip, and much of the old advice is obsolete. Three places still care: the AD converter, which clips at 0 dBFS; the DA converter on the way out; and plugins modelling analog gear, which colour the sound differently depending on how hard they are driven. The usual rule of thumb is around −18 dBFS into those.