The green absorption shield is your starting magic ability. When activated, it lasts a few seconds and will complete absorb any incoming damage. You can use that spell immediately if you hit a green magic orb, but it also recharges by itself. The amount of its charge is displayed on the bar on the right side. You can cast it after it has been fully charged. After gaining new magic spells, "equip" them on your spell list to make one your default spell when your mana is fully charged. Consider doing this with "Heal", as it will help save you in tough situations.