What is an upright rectangle?
Most likely it means an axis-aligned rectangle, i.e., a rectangle whose bottom edge is horizontal (parallel to the $x$-axis) and whose left edge is vertical (parallel to the $y$-axis).  This is the standard meaning of "bounding box".  For future reference, before asking it is useful to check Wikipedia and search thoroughly.  In this case you would have found lots of information on the phrase "bounding box": see, e.g. https://en.wikipedia.org/wiki/Minimum_bounding_box and https://en.wikipedia.org/wiki/Minimum_bounding_rectangle