Chapter 2, Sutra 1.
Tapas, Swadhyaya and Ishwara pranidhana constitute towards Yoga.
Tapas means to burn and refers to the burning of a mental seed. It is the driver behind self-discipline and can be achieved in different ways. Asceticism or fasting destroys ones desire to indulge. Pranayama and Hatha Yoga can also be used to bring heat into the body to cultivate self-discipline. These practices are usually not very pleasant but ultimately bring great benefit to the mind.
Swadhyaya is self-study. That means there should always be a curiosity to study all the layers of what makes 'you', these include the physical structure, mental and spiritual.
Ishwara Pranidhana is to surrender to true self within you. A part which was with you before birth and will remain after birth- pure awareness. It has no allegiance with any matter or being, just awareness in isolation.