Here we have a Process Color business card, which is 3.5" x 2". The print margin for this product is 3.25" x 1.75". However, as you can see, the actual size of the artwork is less than this:

 

If this image were uploaded to the system, it would be resized in order to fit the product's 3.25" x 1.75" print margin. This is probably not the desired effect:

 

In order to accommodate artwork whose size is not that of the product's print margin, the "white space" (aka the "canvas") around the artwork needs to be enlarged to match the print margin.

 

This can easily be done with a tool such as Adobe Photoshop. Just choose the "Canvas Size..." option from the Image menu and enter the new canvas size:


 

Afterwards, you can see that the new size of the artwork will match that of the product's print margin: