Pretty URLs are purely structural URLs that have no query string. This kind of URLs remains same for years which make website more stable and use full. Many e-commerce sites have vast database of products which makes product listing pages URL like this: www.your site.com/product/950/bh/658468/ref=mp_ed_vx This kind of URLs do not provides you any favor, such kind of URLs does not give search engine any indication of what kind of product your visitors are searching for.
Now, it is not a difficult task to make your product’s URL Pretty by using different extensions available in market, if you are using PrestaShop based platform. FME’s PrestaShop Module is one of the best modules for making your products URLs clean and pretty. This FME's PrestaShop Friendly URL without ID module dynamically generated IDs Module automatically eliminates IDs etc. from URLs.
Here in this image you can see the difference of Pretty URL and Ugly URL, from here you can see clearly how you can make your URLs Pretty. The number 5 which is shown in ugly URL is removed in Pretty URL.