The hip hinge is the fundamental of hip movement. The goal is to flex the hip without bending the spine. In layman’s terms: bend over without rounding your back. The hip hinge is important in a host of common exercises including: deadlifts, squats, lunges, hyperextensions, goodmornings, ab rollouts, mountain climbers, and many more.
As you can see in the video, when you bend over there are two ways to do it. You can move through the hips or through the spine. When you move through the spine it curves, which puts uneven and potentially damaging pressure on structures in your back. When you move through the hips, the spine stays straight and the pressure stays even. The large muscles of the hips and upper back are then able to do the work.