Hi,
A bounding box in MATLAB consists of a 1x4 row vector with the following notation [xmin, ymin, width, height]. In the code snippet that you have mentioned, bboxeye(1,3) ie, x_delta seems to be the width of the bounding box representing the eye. Similarly, bboxmouth(1,3) seems to be the width of the bounding box representing the mouth. In the notation, I mentioned above, xmin, ymin represents the coordinates of the lower left corner of a bounding box.
Hope this helps!