The Yoga Sutras of Patanjali is an ancient philosophical text. Sutras are short phrases addressing a particular topic, in this case yoga. The second sutra provides a definition of yoga: "yoga chitta vritti nirodha". Though there are variations on how this statement is translated from the Sanskrit, one common interpretation is "yoga is the cessation of the fluctuations of the mind." In other words, you are doing yoga in order to achieve a stillness and clarity of mind.