What is pranayama? In practice, a Pranayama is a breathing exercise. More than that, in the tradition of yoga, the word prana relates to “life force energy”, or the subtle energy that circulates through the body (or sometimes gets blocked!) to nourish the tissues and functions in the body that keep us alive and healthy. Yama suggests a sense of structure, as in control or restraint. Putting all of this together, we get the sense that in some way, pranayamas are breath-control exercises that are designed to help us stay healthy.
Search the Web or take a look at your favorite yoga book for plenty more detail.
B.K.S. Iyengar‘s “Light on Pranayama” is a classic.