Grading is usually distinguished from correction. Correction is technical: neutralising casts, matching shots from different cameras, balancing exposure so a sequence feels continuous. Grading is interpretive — deciding that a scene should feel cold and clinical, or warm and nostalgic, and building that look deliberately.
The tools work in tonal ranges. Lift, gamma, and gain address shadows, midtones, and highlights separately, so you can push shadows blue while keeping skin neutral. Secondary tools isolate specific hues or regions — pulling a key on skin tones, or on one jacket in one shot — without disturbing everything else.
Good grading is usually invisible, which is why the famous examples are the ones that overreach. The teal-and-orange convention became a cliché precisely because it exploits the natural contrast between skin and sky. The prerequisite is a calibrated monitor: grading on an uncalibrated screen means every decision is made against a lie.