Also referred as the extensor spinae and more commonly known as the lower back muscles. A combination of tendons and smaller muscles, it is not a single muscle on it's own, even though it is a large covered area of the back.
Your lower back muscle region is a significant aspect of your core, which also includes your abdominal and oblique muscles. Having a strong lower back can have a significant improvement on lower back strength, pains and aches. Since you use your lower back on a daily basis, having strength in that muscle will great improve everyday tasks such as lifting or bending over.