A chill is an object used to promote solidification in a specific portion of a metal casting mold. Normally the metal in the mold cools at a certain rate relative to thickness of the casting. When the geometry of the molding cavity prevents directional solidification

Directional solidification

Directional solidification and progressive solidification describe types of solidification within castings. Directional solidification describes solidification that occurs from farthest end of the casting and works its way towards the sprue...

 from occurring naturally, a chill can be strategically placed to help promote it. There are two types of chills: internal and external chills.
Internal chills are pieces of metal that are placed inside the molding cavity.