It is the cooling system for the underhood AC/DC inverter. If it is down on coolant, check for leaks. If the inverter dies, it is big bucks. Prius has an electric power steering motor. There is no power steering fluid in this car.
I called guy who works for Toyota and found that it is special Toyota coolant for synergy system and you can only use Toyota coolant because something about electricity passes thru to make system work and you can ruin inverter and system