Samkhya-Yoga-Kundalini
Sva-viṣaya-asamprayoge cittasya sva-rūpa-anukāra iva-indriyāṇāṁ pratyāhāraḥ
Resembling the nature of the mind, as if the senses are not connected with their objects, is pratyahara.
Tataḥ paramā vaśyata-indriyāṇām
From that, [the yogi gains] supreme control of the senses.
This is the traditional translation of these two sutras. It is explained with the analogy of a queen bee and the worker bees. When the queen bee leaves the hive, the worker bees follow, and where she rests, the worker bees also rest. Here, the queen bee is a metaphor for citta, and the worker bees are the senses.
Most people have experienced the phenomenon of being so absorbed in something that they fail to notice what is happening around them. The senses are always functioning, but it is the contact of the mind or awareness that actually causes us to perceive the sensations that they receive. Pratyahara is taken to mean turning the mind inward, as if it has been detached from the senses and their objects.
There is not necessarily a sequential progression from pranayama to pratyahara to dharana, the next limb. Dharana can be and often is performed on perceivable objects. In that case, pratyahara obviously could not precede it. But at some point in the process of yoga, the yogi must turn away from the senses and their objects and focus on the inner layers of consciousness. Pratyahara is the transition from the external practice of yoga to the internal practice.