The attainment of Samadhi is the ultimate step in Ashtanga Yoga. This is the zenith of all yogic endeavors—the ultimate 'yoga' or connection between the individual and the cosmic intelligence.In Samadhi yogi become one with the True Self and merge into the object of concentration. And true unity of all thought and action takes place.
At the moment of samadhi our ego dissolves and we become one with the Divine Entity. The mind can’t distinguish between self and non-self, or between the object contemplated and the process of contemplation. The senses get transcended by complete refinement of the body and mind. The supreme and eternal liberty of a soul is beyond all stages and beyond all limitations of time and space. The eight "limbs": Yama, Niyama, Asana, Pranayama, Pratyahara, Dharana, Dhyanaa and Samadhi are the stages of the gradual withdrawal of consciousness from outward world.
Yama and Niyama, which are ethical obligations, Asana, Pranayama, which is breath control, Pratyahara, which is sense withdrawal, Dharana, which is concentration, Dhyana, which is meditation, and Samadhi, which is the experience of unity with God.
The act of meditation leads to the attainments known as Samadhi. But to experience Dhahrana and Dhyana, the mind must first be stable, calm cool. If the mind is busy responding to external stimuli, it cannot enter into the auspicious circle of Dhahrana. It may get you into trouble, if you deliberately force for Dhahrana when your mind is not prepared for it. Therefore the Yoga Sutra suggests the practice of asanas and pranayama as preparation for Dhahrana, because these influence mental activities and create space in the busy schedule of the mind. Once Dhahrana has occurred, Dhyana and Samadhi can follow.