The Yoga Sutras is about 195 aphorisms in four short chapters, and it is famously terse: sutras are memorisation aids and were never meant to be read without a teacher or a commentary. The second one states the whole aim: yoga is the stilling of the movements of the mind. When those movements stop, the third adds, the seer rests in its own nature.
The metaphysics behind it is broadly that of Samkhya, with a fundamental division between purusha, consciousness, and prakriti, everything else including thought and personality. Suffering arises from mistaking one for the other, and liberation, kaivalya, is the disentangling. The path is set out in eight limbs: restraints, observances, seat, breath regulation, withdrawal of the senses, concentration, meditative absorption and samadhi, with the third chapter cataloguing the extraordinary powers that arise along the way and warning that they are obstacles.
One point is worth stating plainly because the modern association is so strong. This is not a text about physical postures. Asana means seat, it gets three aphorisms, and what they require is that the position be steady and comfortable so that it stops being a distraction. The sequences of postures that the word yoga now denotes in most of the world are a later and largely modern development, and they cannot be found in this book.