It is the science of physical and mental control. A great sage, Shri Patanjali, pioneered it. It is also known as Astanga yoga, or the yoga of eight steps. All eight steps are important and are to be followed sequentially, leading to the ultimate realization of Truth or God. The first two stages are meant for moral purification, without these no spiritual progress is possible.
Rhythmic breathing helps concentration of the mind.
This mental exercise gradually slows the rush of thoughts from within to the surface of the mind. Now the mind has become ready for concentration on one object or on one idea. In the sixth stage, a person should concentrate the mind either on one external object or one internal idea upon which to meditate. One finds that, in spite of the best of efforts, the mind does not remain glued to the chosen object. The object appears too hazy and there are breaks in concentration.