Canonical links help search engines consolidate the information they have for the individual URLs into a single, authoritative URL. Therefore, if you syndicate your content for publication on other domains, canonical links help to consolidate the page ranking to your preferred URL.
Note that canonical links should only be applied if you actually own the original content. If you are copying someone else's work, you should not apply a canonical link. (Members who do this are in violation of the AUP.)
By supporting canonical links, however, we allow authors to assert that they are the owners of content that may be published elsewhere.
You can add a canonical link for your post when you create or edit your post.
If you embed content from YouTube or other sources, does that require a canonical link?
If you are embedding content from another site as part of your post using the built-in embed tool in Narrative, that does not require adding a canonical link for your content, because the content has been authorized to be embedded by the third party source.
You should only worry about adding a canonical link to your post if you are copying an entire post that you yourself previously made on another site.