What is concentration in Namaaz

In Safinat’ul-Bihār, volume 2, Page 145 it is mentioned that on the way to a battle, the Messenger of Allah (S) encamped at a spot to spend the night. The night watch was entrusted to Ebad bin Bushr and Ammar Yasir. It was agreed that Bushr would vigil the first part of the night and, Ammar the other half. So Ammar Yasir went to sleep and Bushr was guarding the camp. The infidels decided to attack the sleeping Muslims.  An attacker moved stealthily and came near the camp but he saw Bushr standing in prayers. However, Bushr was so still that the attacker could not understand whether it was a tree, an animal or man. To make sure the infidel shot an arrow towards Bushr. It pierced Bushr but he continued to recite the prayer with the same composure. The second arrow was shot but Bushr didn’t show any sign of discomfort. Finally the third arrow struck and Bushr shortened his prayers and awoke Ammar.  Ammar awoke to find Bushr smeared in blood. He asked why he had not woken him at the first arrow. Bushr replied, “I was reciting Surah al-Kahf in the prayers and I didn’t like to discontinue. And if there had been no risk to the life of Prophet (S) and the Muslims and fear of dereliction of the duty entrusted to me I would have continued my prayers till I were dead.”  Reference : Greater Sins Volume 3