What is a canonical meta tag?
A canonical meta tag specifies a canonical, or preferred, address where a page can be found. On larger websites with a large amount of dynamic content, it's quite possible that the content from a page can be found simultaneously on different URLs. For example if the site uses URL rewrites in the paths (example: /products/shoes_size_45.html), but at the same time that same detail page is also requested using URL variables (example: /products.html?id=55).
If the same page content can be found on more than one URL, there's an increased risk of duplicated content, that is to say that the search engine automatic indexers detect that the content on a particular web page has already been found on another page. In cases of duplicated content, the website in question could be sorted lower on the results page of the search engine (SERP).
By inserting a canonical meta tag, it becomes clear for the bots what the basic address of the webpage's content is. Possible aliases, pages where the content can also be found, are not stored by the search engine. This results in a uniform registration of unique website pages in the search engine.