Yoga Sutras 2.50
Patanjali, explained by Swami Vivekananda
बाह्याभ्यन्तरस्तम्भवृत्तिः देशकालसंख्याभिः परिदृष्टो दीर्घसूक्षमः ॥
bāhyābhyantarastambhavṛttiḥ deśakālasaṃkhyābhiḥ paridṛṣṭo dīrghasūkṣamaḥ
Its modifications are either external or internal, or motionless, regulated by place, time, and number, either long or short.
In plain words: Breath control (Pranayama) has three movements: breathing in, breathing out, and holding the breath still. Each can be shaped by where in the body you focus, how long it lasts, and how many counts you use.
Swami Vivekananda explains
The three sorts of motion of Pranayama are, one by which we draw the breath in, another by which we throw it out, and the third action is when the breath is held in the lungs, or stopped from entering the lungs. These, again, are varied by place and time. By place is meant that the Prana is held to some particular part of the body. By time is meant how long the Prana should be confined to a certain place, and so we are told how many seconds to keep one motion, and how many seconds to keep another. The result of this Pranayama is Udghāta, awakening the Kundalini.