Protection lowers the targets armour class by 1 full point for a while. This means it is harder for opponents to successfully strike the spells recipient. There is one anomoly in player AC which some do not appreciate. There are times when someone with AC 0 casts protection and nothing seems to happen. It has, but you have moved from +0 ac to -0 ac. If you had started on AC -0 then you would have gone to AC -1. The net effect is that for anyone on AC +0 the protect spell has minimal effect.