Gain staging is the practice of managing volume levels throughout the recording and mixing chain — from microphone preamp to channel fader to plugin input to master bus.
The goal is to keep signals in the 'sweet spot': loud enough to stay well above the noise floor, but quiet enough to avoid clipping (digital distortion). In a 24-bit DAW, the sweet spot is typically around -18 dBFS to -12 dBFS.
Poor gain staging is one of the most common mistakes in home recording. Signals that are too hot clip and distort; signals that are too quiet get buried in noise when amplified later.