Union over intersection

I have a difficulty in understanding the following points?? Can someone help me out in this???

You should do as they suggest and draw yourself some pictures of the various possibilities to understand the geometric interpretation.

One key point to realize is that the way the coordinate system works for images is a bit different than normal geometry:

(0,0) is the upper left corner.
As the x coordinate increases, you move to the right.
As the y coordinate increases, you move down.

When they give the coordinates for one box, the first two values are the (x,y) of the upper left corner and the second two values are the (x,y) coordinates of the lower right corner.

With all the above in mind, what they describe should make sense now. And it really helps to draw pictures.