Revitalize is one of the first spells you will learn. It allows you to heal yourself or other players.
You learn the ability from a Scroll of Revitalize. One is given to you by Mage Taurnik during the quest  Basic Magic. (If somehow you should lose it, you can also buy one from a Combat Trainer for 10 silver.)
The ability is categorized as a Prophet ability, but most players will have it learned since it was a quest reward. With its low mana cost and short cast time, this skill is very useful no matter what class you are playing and whether you use magic or not.