Vitality is one of the 4 stats that may be improved upon using attribute points earned from gaining levels in game.
Choosing to raise your Vitality stat will improve your character's overall health, improving the amount of damage you can suffer before dying.
Vitality is a versatile stat, meaning all classes have a very general use for it. When your character's health drops to zero, you will die, and you will resurrect with 50% of your health at the nearest leystone you have discovered, or with 100% of your health at the place where you died if using a resurrection idol.