Super-duper antioxidant & stabiliser. Some suppliers flog weird bases containing vague ratios of various chemicals and solvents, but I prefer to just send the actual workhorse material and let you incorporate without the hand-holding.
 
It's worth noting that this material works precisely because it sacrifices itself for the good of your perfume - the molecule literally gets used up and offers itself up for oxidation rather than your precious perfume materials. With that in mind, do be a little wary of super cheap soapmaker's Vitamin A-style products: if they aren't stored right and aren't themselves kept in an oxygen-free environment, there's every chance they can arrive at you already partially oxidised.