A core is a device used in casting and molding processes to produce internal cavities.
Think of a core as the “placeholder” for empty space within a casting. If the pattern is a replica of the object to be cast, the core is the replica of the cavities within that cast object. The core is typically made of chemically bonded sand and is destroyed to remove it from the casting.
A core box is the tool which a foundry uses to form a core. In simplified terms, the core box is a reusable “mold” that foundries use to make cores.