Vira means brave. The way a brave man takes position while attacking his enemy, the similar position is formed in this asana, hence it is called as Virasana.
In this asana the joints of the legs, the waist, the spinal column and the neck get curved in opposite direction. As a result of this, the blood circulation to these joints is regulated. The spinal column becomes elastic and its functioning improves. There is pressure on the digestive organs and the belly gets stretched, which promotes their functioning.