A canonical URL is the URL of the best representative page from a group of duplicate pages, according to Google. For example, if you have two URLs for the same page (for example:
example.com/dresses/1234), Google chooses one as canonical. Similarly, if you have multiple pages that are nearly identical, Google can group them together (for example, pages that differ only by the sorting or filtering of the contents, such as by price or item color). (You might hear the term "canonical page" used occasionally, but that is technically incorrect, as it is a specific URL that is actually canonical.)
The canonical can be in a different domain than a duplicate (such as en.example.com and fr.example.com).