The canonical link is used to tell the major search engines which URL you would prefer them to use when you have more than one URL pointing to the same page.
Here are some examples of the sorts of URL’s we are talking about.
www.example.com/product.php?name=shoes&color=black&brand=clarks
www.example.com/product.php?brand=clarks&color=black&name=shoes
If you simply add a <link> tag to specify your preferred version of [...]