An asana is a posture. A body can take many postures but only few are called yoga asanas because of the movement and balancing of energies in the body. There are many ways to reach the highest level of consciousness but starting with the body is the easiest way. When you get mastery over the physical body using asanas then the ground is prepared for you to go to the next level of consciousness and energies.
In the eight limbs of yoga after the discipline (yama & niyama) Patanjali Maharshi starts with Asanas. According to him, body is the very foundation stone and if you don’t get mastery over the body, the higher training will not be possible.
He says, the Asana should be steady, comfortable and blissful. Asana is the grace of the body and only in a graceful body a graceful mind exists; and only in a graceful mind does a graceful self becomes possible; and only in a graceful self, the divine can enter. For the same reason after Asana, comes pranyama, prathyahara, dharana, dhyana and samadhi.
The science of asanas is called Hatha Yoga. ‘Ha’ means Sun and ‘Tha’ means Moon. Everything in this existence is bipolar or dual in nature. Feminine and masculine, sun and moon, shiva and shakti, yin and yang, whatever you call, they represent duality. Even body and mind are two aspects of same phenomena. Whatsoever happens in the body affects the mind and whatsoever happens in the mind affects the body. The aim of yoga asanas is to bring balance between dual energies. Then only one can enter higher levels of consciousness.
Asana which is comfortable for one person may not be the same for other because everyone’s body is unique. There are 84 basic asanas through which one can elevate his consciousness. These are not just postures but 84 ways of possibilities. Out of this, if one can master even a single asana, then dualities and the disturbances caused by dualities will disappear.
If we look with deep scientific perspective, Asana is a geometrical alignment between the macrocosm (bramhanda – universe) and microcosm (pindanda – you). Performing Asana is a dynamic meditation. One has to be more mindful when Asana is practiced. One’s Physical energy (body), Mental energy (mind) and Pranic energy (breath) should be in sync with each other. Once you master any one posture (Asana siddhi) you can feel the subtle connections between you and the universe. Then you can download the whole cosmic energy that is just around you always.