This box is used when you want to decide where about you want your image to be mirrored at.

Here are a couple of examples of my final outcomes.

The steps i followed were pretty simple. I started by deciding which image i was going to edit first. Then by hovering over the image tab at the top and press canvas size i was able to drag the photo from one side to the other using the small arrow in the corner. After having selected it and by pressing control J.

Here is another example of the same image i edited but in black and white.