When adding an item into the shopping cart, you will see crop lines if your photo doesn't fit.
When you click on View Cart you will see a new view appear where you can select Edit Crop:
The Edit Crop screen will allow you to move the border of the print and decide where the crop should appear. This option will only be available if you are adding a print product to the Shopping Cart which is in a different aspect ratio than the selected image file.
Some print products, like the Gallery Wrap canvas also have a border requirement to display a preview of where the image will wrap around the frame of the canvas. When these products are added to a Shopping Cart, extra wide crop lines will be displayed for the "wrap".