The stars are a reflection of the quality of your images. 5 stars mean your images are the best possible for printing. Your wallet should look very much like the rendered image you see on your screen when you complete the design process.
Anything less than 5 stars is not ideal and may result in some grainy printing results.
We strongly recommend you to only use 5-star images and warn you that images with less than 3 stars will be seriously distorted upon printing, so please proceed at your own risk. Below 72 DPI (0 stars) is definitely not the way you want to express yourself on your wallet, believe us.