The muscles running down the back of the body, mainly the glutes, hamstrings, spinal erectors and calves, which work together to extend the hip.
It is the engine for everything that involves standing up, jumping, sprinting or picking something off the floor. Most desk-bound clients arrive with it undertrained relative to the quadriceps, because sitting shortens the hip flexors and nothing in daily life asks the glutes for much.
Two lifts cover it: a hinge (Romanian deadlift, conventional deadlift, hip thrust) and a knee-flexion movement (leg curl, Nordic curl). One of each, twice a week, is the baseline.
A rough check on balance: a client should be able to Romanian deadlift for reps at a weight close to what they back squat for the same reps. If their squat is 100 kg for 5 and their RDL is 60 kg for 5, the back of the body is the limit.