What does this report contain?
Pages with URLs that are different to the canonical URL specified in the canonical tag in either the HTML or HTTP header.
How can this report be used?
If the canonical URL is different to the URL of the page (canonicalized), it tells the search engines not to index the URL of the page in favour of the canonical URL and effectively makes the URL non-indexable. All canonicalized pages should be reviewed to ensure the canonical tag has been implemented correctly with the preferred URL in all instances.