Pranayama is the practice of breathing in Yoga. There is a certain breath that is recommended, and it sounds like you're whispering with your mouth closed.
It is helpful to concentrate on pranayama while practicing because it can draw the focus inward. It can be tempting to peek at your neighbor while practicing Yoga, but that contradicts the point of practicing! Yoga is about what is happening with your mind and body on your mat.
I found an article that details the purpose and benefits of pranayama that is helpful for even veteran practitioners of Yoga. It comes from ABCs of Yoga's website.
■Pranayama teaches us the proper way to breathe. We become used to breathing from our chest, using only a fraction of the lungs, not knowing that this unhealthy and unnatural way of inhaling may lead to several complications. With yoga breathing, we increase the capacity of our lungs, bringing more oxygen supply to the body to function well. We learn how to breathe slowly and deeply - the right way.
■Pranayama reduces the toxins and body wastes from within our body. It prevents one from acquiring diseases.
■Pranayama helps in one's digestion. With the proper way of breathing, one's metabolism and health condition will start to improve.
■Pranayama develops our concentration and focus. It fights away stress and relaxes the body. Controlling one's breathing also results in serenity and peace of mind.
■Pranayama offers a better self-control. Through concentration, one can better handle temper and reactions. Mind can function clearly, avoiding arguments and wrong decisions. Moreover, self-control also involves control over one's physical body.
■Pranayama leads to spiritual journey through a relaxed body and mind.
If you'd like to know more, head over to read the full article here.