A canonical link references the original publication URL (on your website) so that Google knows which was the original piece and which was the syndicated one. This is very important to your SEO. Absolutely DO NOT let any other website or publication syndicate your content without properly attributing your work with a canonical URL.
Here's some info on that: https://moz.com/learn/seo/canonicalization
I recently had a publication syndicate one of my posts. Beforehand, I had them agree to reference my original post with a canonical link. And they agreed. But then they did not follow through. And I had to be the one to educate them about how to add a canonical link. To this day, their webmaster still has two canonical URLs for the same post: (1) one for their URL which their CMS created automatically (2) and one for mine. That was the best I could get him to do. But I was pissed. Especially because my only two stipulations for syndicating the post was (1) a canonical link to the original publication URL, and (2) a do-follow link back to my website. They agreed to both and did neither until I got on top of them.
So now, moving forward, anyone who wants to syndicate my work, I require that they agree to those terms (just like I did last time). And additionally I have them show me through example that they have the ability to properly add a single canonical link back to the original domain on a previous article.
I recommend you do the same.
