Albert Barnes Commentary 2 Kings 6:17

"And Elisha prayed, and said, Jehovah, I pray thee, open his eyes, that he may see. And Jehovah opened the eyes of the young man; and he saw: and, behold, the mountain was full of horses and chariots of fire round about Elisha." — 2 Kings 6:17 (ASV)

Open his eyes that he may see - Elisha’s servant lacked the faith of his master. Elisha therefore prays that he might be given a vision of the spiritual world and see, as if with his physical eyes, the angelic host that he himself knows to be present.