What it Means:
Wikipedia: Asana originally meant a sitting position. In the practice of Yoga it denoted the art of sitting still but later was applied to any posture useful for restoring and maintaining a practitioner's well-being and improving the body's flexibility and vitality, cultivating the ability to remain in seated meditation for extended periods. Such asanas are known in English as "yoga postures" or "yoga positions".
In the Yoga Sutras, Patanjali says that the only requirement for practicing asanas is that it be "steady and comfortable". The body should be held poised with the practitioner experiencing no discomfort. When control of the body is mastered, practitioners are believed to free themselves from the duality of heat/cold, hunger/satiety, joy/grief, which is the first step toward the attachment that relieves suffering. This non-dualistic perspective comes from the Sankya school of the Himalayan Masters.
There's only 84 original yoga asanas, so most of the poses we practice today were created recently.