the search engine friendly URL that you want the search engines to treat as authoritative. In other words, a canonical URL is the URL that you want visitors to see.
The Canonical tag should be implemented in category pages that display a certain number of products and are spread amongst multiple pages. If your page is for instance /category.html and their are multiple pages within the category i.e. /category.html?page=2, /category.html?page=3 the canonical tag should be useds within all of these parametrised pages to inform Google that they are all duplicates of /category.html and so they will not be seen as having duplicated content on all of these category pages. This ensures that your correct page is served up within the SERPS.