Use of the term Grain in regard to metal is the individual crystals in the metal.

The grain structure is the relationship between the small, individual crystals in a metal or alloy. Between the grains are grain boundaries.

Intergranular Corrosion occurs at the grain boundaries in austenitic stainless steels that have been heat treated between 850° and 1450°F.
Usually caused by precipitation of the chrome carbides.
Proper heating and cooling of the stainless during casting and forging are vital to prevent this type of problem.