Pranayama is divided from two Sanskrit words 'Prana' and 'Ayama'. Prana means universal life force energy and Ayama means elongation or to regulate. Pranayama are breathing exercises which clear the physical and emotional obstacles in our body to free the breath and so the flow of Prana - life energy. Through regular and sustained practise of Pranayama you can supercharge your whole body.
Acceding to Patanjali's eight fold path - fourth step is;
Yoga after having the physical discipline, proceeds to teach how to regulate and control the Prana. It's called the discipline of prana or bio-energy. Actually Prana is the vital force through which life is sustained.
In the Yoga Sutras some commentators used the term prana as meaning the breath. Actually Prana is neither the breath nor the air. It would rather be the bio-energy that causes the motion of the lungs, that the vital element in the Universe, and life is sustained exclusively because of it.