Prana means life source, life energy or vital energy.

We should not mistake prana for breath what we usually inhale and exhale through nostrils.
But by regulation of our breath through pranayama or breathing exercises we can feel and use the life source or life energy or vital energy in abundance.

Although asanas produce similar effects in the body, pranayama affects rhythmic expansion of lungs creating the proper circulation of the bodily fluids within the kidneys, stomach, liver, spleen, intestines, skin, and other organs. The lungs are directly concerned with the disposal of carbon dioxide in the blood vessels and preventing ammonia, ketones and aromatic amines from building up toxic levels.

The lungs need to be kept clean and free from bacterial diseases by an efficient circulation of blood and lymph. Pranayama helps here by keeping the lungs pure and by increasing the flow of fresh blood.