In heraldry, a compartment is a design placed under the shield, usually a landscape or wreath of some sort upon which the supporters are depicted as standing. It is a comparatively late feature of heraldry. A compartment might be purely an artistic feature, or (typically later) form an actual part of the blazon.
Usually when arms are augmented by supporters, a compartment will be added too. In rare cases, a compartment might be granted as an augmentation.