A bounding box, often abbreviated into BBOX, is a list of the extreme coordinates of a geometry. Bounding boxes play a big role in spatial queries, as they allow for fast coarse computations; If two geometries' BBOXes do not intersect, there's no point in wasting CPU cycles for intersecting them precisely. In PostGIS, bounding boxes are computed and cached internally. There are specialized data types for bounding boxes: box2d, which can be visualized as a rectangle, and box3d, which forms, well, a box.