This section identifies pages that contain a canonical tag and the canonical URL. The canonical tag goes in the page’s <head> and indicates that the page hosts content that is copied, or similar enough to be considered duplicate, from another URL.
Canonical URLs are an important part of managing duplicate content that naturally arises from large, complicated websites. The canonical tags show search engines where they can find the original version of the page, and where they should consolidate link juice. Note that canonical tags, while strong suggestions, are still just suggestions.