A Canonical-Tag is an HTML element used to tell search engines which URL version of a page should be considered the preferred version. This is important when there are multiple versions of a page that all have the same content but are accessible at different URLs. Using a canonical tag ensures that search engines index the preferred version of the page and display it in their search results. This prevents the content of the page from being treated as duplicate content and having a negative impact on the ranking. The canonical tag is usually placed in the head area of the page.