A method used to soften a metal by heating it to a predetermined temperature somewhere below its melting point. Annealing is also used to take the stress out of a metal.
The stress could be residual from the manufacturing process or as a result of work hardening by beating on, or flexing the metal part.
Stainless steel is sometimes annealed after welding in an attempt to shake back some chrome from the area that was not in the heat-affected zone.
In the heat affected zone the chrome was depleted as it formed chrome carbide when the carbon in the metal combined with the chrome during the high temperature welding process.
You can learn more about this if you look up Inter granular Corrosion.