The crown is the part of the barrel immediately inside of what would be called the "business end" of the gun in question. It's the terminal end of the barrel, right where the bullet exits the barrel and relies upon kinetic energy to fly onwards.
Over time, particularly due to improper cleaning habits, it is possible for a crown to become damaged to the point where it affects accuracy. Extreme examples will cause a bullet to actually "keyhole" sideways through the target, leaving behind a perfect sideways outline of itself. The only possible fix for that is to recrown the barrel.
-- SeanNewton - 30 Dec 2005