Combat Points (CP) add an extra layer of protection when being attacked by another player. CP acts as a buffer that must be depleted before damage will affect the player's health points.
If a player is damaged by another player, CP is consumed first with no effect on HP. If the CP is not enough to consume all the damage, HP begins to be consumed.
CP has also a significant role in the damage calculation.
CP can be refilled with War drink. To refill your CP go to Possessions -> Inventory -> Drinks -> War menu, select the desired food and press Drink now.