Yoga Sutras 2.49
Patanjali, explained by Swami Vivekananda
तस्मिन् सति श्वासप्रश्वासयोर्गतिविच्छेदः प्राणायामः ॥
tasmin sati śvāsapraśvāsayorgativicchedaḥ prāṇāyāmaḥ
Controlling the motion of the exhalation and the inhalation follows after this.
In plain words: After posture comes pranayama, which is controlling the breath in and out. The teacher explains that prana is the life energy in the body, and breath control is the easiest way to begin controlling it.
Swami Vivekananda explains
When posture has been conquered, then the motion of the Prana is to be broken and controlled. Thus we come to Pranayama, the controlling of the vital forces of the body. Prana is not breath, though it is usually so translated. It is the sum total of the cosmic energy. It is the energy that is in each body, and its most apparent manifestation is the motion of the lungs. This motion is caused by Prana drawing in the breath, and it is what we seek to control in Pranayama. We begin by controlling the breath, as the easiest way of getting control of the Prana.