According to the lore: "Stimpak, or stimulation delivery package, is a type of hand-held medication used for healing the body. This item consists of a syringe for containing and delivering the medication and a gauge for measuring the status of the stimpak's contents. When the medicine is injected, it provides immediate healing of the body's minor wounds."
Of course, it is a prop, not a real medicine, just in case ;-)
