Every point in this skill stacks with Combat Recovery to provide an additional 0.15 to the % of your combat energy regen. With both skills maxed you will regenerate energy at 30% (35% with a staff and Divination) of your out-of-combat rate when in combat.
Combat Endurance is an Protection skill, that can be leveled starting at character level 20. Raising this skill at or above your character level will incur an experience point penalty. As an Protection skill, Combat Endurance can be raised actively, by sacrificing combat trophies to the Altar of the Shield in town, or raised passively.