Your car has a seperate adblu tank. It's used to help with emissions. The tank is nearly empty and needs to be filled. On my car, you'll find the filling point next to where you fill your car with Diesel (it's the blue coloured cap) Most cars take 12 litres, but some have a 24 litre tank.
You can buy Adblu from most petrol stations - some even have a Adblu pumps, but I've not come accross any