Instance: You might have an e-commerce Web-site with product pages for various measurements of a certain t-shirt.   You specify the first URL because the search engine could have problem figuring out which one particular need to be the primary URL.  The next tutorial will teach you ways to help https://www.presta-addons-modules.com/en/seo/prestashop-seo-automatic-hreflang-tags-canonical-tags-on-all-pages.html