It's called by many names - prana, qi, energy, the force, quantum - but it's all the same. The original cause. The primordial and invisible building block and essence of all life.
Control, regulation and mastery of the breath (prana-yama) is a key step on the yogic path. Yoga literature does not distinguish between "breath-substance," "mind-substance," or "LIFE" substance (prana) The three are so closely linked that there only exists a differentiation in the untrained mind. In reality, breath, thought and life are all the same - physical manifestations of pranic consciousness.
Slowing and controlling the breath will slow and control the random "thought waves" of the mind.
Until the human mind can be mastered, there is no peace of mind. Master the breath and the mind follows. Master the mind and peace will follow.
Om shanti shanti shanti