Inhale, and God approaches you.
Hold the inhalation, and God remains with you.
Exhale, and you approach God.
Hold the exhalation, and surrender to God.~ Sri T. Krishnamacharya
Prana is a Sanskrit word meaning life force energy, and it encompasses all aspects of energy that support life, such as oxygen, the sun, and the food we consume.
The breath has the unique characteristic of being both a subconscious and a conscious function. Because of this, we can enrich our individual life force, or prana, through the conscious control of the breath, which is called pranayama.
A variety of breathing techniques can be used to balance, stimulate or calm the mind and body. The breath forms a bridge between the energies of the mind and body by working on the parasympathetic nervous system. As a result, the pattern of the breath both reflects the state of the mind/body complex, and can alter it as well.
Pranayama is one of the Eight Limbs (ashtanga) of yoga practice, as outlined by Patanjali in Chapter II, section 29 of the ancient foundational text called Yoga Sutra: "yama niyama asana pranayama pratyahara dharana dhyana samadhayo astavangni". In fact, it is so important that it's the only practice explicitly offered in the entire Yoga Sutra.
In Chapter I, section 34, he writes: pracchardana vidharanabhyam va pranasya - "... obstacles can be lessened by forcibly exhaling, then retaining the prana during the pause following the exhalation."
See a simple practice of this breathing technique to quiet the mind quickly and easily.