Yoga Sutras 2.54
Patanjali, explained by Swami Vivekananda
स्वस्वविषयासम्प्रयोगे चित्तस्वरूपानुकार इवेन्द्रियाणां प्रत्याहारः ॥
svasvaviṣayāsamprayoge cittasvarūpānukāra ivendriyāṇāṃ pratyāhāraḥ
The drawing in of the organs is by their giving up their own objects and taking the form of the mind-stuff, as it were.
In plain words: Pratyahara means pulling the senses inward. The senses stop chasing outside things and settle into the mind, so the mind stays calm.
Swami Vivekananda explains
The organs are separate states of the mind-stuff. I see a book; the form is not in the book, it is in the mind. Something is outside which calls that form up. The real form is in the Chitta. The organs identify themselves with, and take the form of, whatever comes to them. If you can restrain the mind-stuff from taking these forms, the mind will remain calm. This is called Pratyahara.