A canonical URL lets you tell search engines that certain similar URLs are actually the same. Sometimes you have content that can be found on multiple URLs, or even multiple websites, but by using canonical URLs, you can have these on your site without harming your rankings.
For example, let's say that you have a Newsroom on Notified, but that you also republish each post on your own website. In this case, you can tell Notified when publishing your post that it should direct all search engine traffic toward your own site.
You'll find the option to add a canonical URL under step 2 ("Settings") when writing a post.