Now, the discipline of yoga.
Atha = now, from this point on
yoga = yoga (see definition in Sutra 2)
anusasanam = instructions to the practice
It is a tradition in the sanskrit literature that a new topic starts with the word 'Atha' and ends with 'ithi'. Yoga is a discipline that involves both philosophy and practice. That is why the text starts with the sentence: now the discipline of yoga. The author says that he begins to explain the philosophy and practice of yoga.
The commentary of Sage Vyasa
Atha denotes the commencement of a new chapter or topic. The science of instructions on yoga is understood as begun.
The commentary of Swami Vivekananda
Now Concentration is explained
The commentary of Sri Osho
Now the discipline of yoga. Discipline means the capacity to be, the capacity to know, the capacity to learn. We must understand these three things. What is discipline? Discipline means creating an order within you. As you are, you are a chaos. As you are, you are totally disorderly. ”Yoga is discipline” means yoga wants to create a crystallized center in you.