The King's Champion was the man chosen to deal with the affairs of the mortal plain, as the holder of the kingship had too many religious ties to be directly confronted.
Generally speaking, the champion was the king's best warrior. In Irish myth, Cù Chulainn was Conchobar's champion. In Arthurian legend, the champion was Cei/Keii/Kay.
In the hall, the champion would get a special portion of the food, usually the best. In battle, he would be expected to be the fiercest of the attackers. If there was a challenge to the king, the champion was expected to meet it.