Constitution

Constitution also known as health and formerly as Hitpoints or HP is a skill that affects how many life points (LP) a player or monster has. Life points represent the amount of damage a player or monster can withstand before it dies. Death occurs when a player or monster's life points reach zero.

Skills such as Constitution, Attack, Strength, Defence, Ranged, Magic are combat skills.

You may train then via Slayer or fighting various monsters.