A shaman is a tribal healer who can act as a medium between the visible world and the spirit world.
Shamans are kind of a mix between priests and doctors. Like a priest, a shaman is a holy man who represents a religion — in this case, shamanism. Like a doctor, a shaman heals people — or at least claims to do so. If you're not into shamanism, you can use this term more generally: A really good teacher, especially one who works a little mysteriously, might be said to work "like a shaman".