This is a process similar to annealing, where metal is heated to a high temperature and held at this temperature for several hours to improve grain structure. But unlike with annealing, where the metal is cooled slowly in the furnace, it is cooled more swiftly by removing it from the furnace to cool in air.
This makes the metal stronger and harder than it would be after annealing, and because of this the normalizing process is often employed to treat steel plate used for pressure vessel fabrication. Steel casting, forging and rolling are both processes that might produce grain structures in the metal that require normalizing before it is used or further processed. And for more detail click and see wikipedia entry.