Urea, or diesel exhaust fluid, is a substance the car injects into the exhaust. It reacts with and neutralizes nitrogen emissions. It's required in the US and Canada because of emissions regulations, but is not present in most cases elsewhere in the world. Although pretty soon it'll start appearing in Europe as well.
To the OP, that warning means you need to top off the urea fluid or the car will not restart after 900 miles.