The Safe Area is highlighted with a green dotted line when viewing your product in the "Design View."
For best results, keep text and other important parts of your design, image, or photo INSIDE the Safe Area!
In the example below, the letters need to be placed within the dotted green line to ensure that it is included completely in the final product.
"Bleed" is a printing term used to indicate the area that will be trimmed off after the job is printed and cut down to the finished size. When using images, the best strategy is to fill the entire design area of the product to ensure that you do not have any unwanted borders framing your image. This ensures that even after trimming or cropping, the background is printed to the full edge of the product.
Just remember to make sure any important parts of your design fall inside the Safe Area and that your background image (if you are using one) covers the entire available design area.