Flutes are raised beads on a piece of armor. Flutes serve the function of making armor much more ridged than a un-fluted piece. Historically flutes are more commonly found on later pieces. The late German gothic armor being some of the most fluted armour ever produced.
Fluting by HandEdit
Fluting is typically done by hand, with a fluting tool, usually a chisel with the edge rounded so as to not cut the metal.