The gluteus maximus refers to the outermost muscle of the three glutei making up the buttocks. It is the largest muscle in the gluteal group, located closest to the skin's surface, and is responsible for extending the hips. The gluteus maximus plays a crucial role in activities such as walking, running, and climbing stairs, providing power and stability to the lower body. It is one of the key muscles involved in maintaining an upright posture and supporting the body's weight.