What does "down fired" mean?

By placing the burner at the top of the boiler and pointing it downward, the water that is closest to exiting the system is heated first, quickly supplying hot water in the home. The downward pointing burner/blower also serves to remove condensation from the cast iron jacket inside the boiler, preventing corrosion from occurring. With rugged cast iron construction to help muffle the noise, having the burner/blower facing down into the body of the boiler helps make Ray one of the most quiet operating boilers on the market.