Series: web tech terminology explained for marketing professionals
Large websites typically have a lot of duplicate or at least fairly similar content. When a search engine indexes the website, it bumps into a lot of individual pages which compete for results on same search keywords.
A canonical URL is a technical measure for telling the search engines that "even though there is a lot of duplicate content on our website, THIS is the page that we actually would like you to consider for search results".
Let's take an imaginative company that manufactures hydraulic presses as an example. On their website they have a product category page "Hydraulic presses" which contains a lot of quality content on the topic and links to individual product pages. On the three product model pages (X300, X600 and X1000) they have model specific technical data AND a lot of generic content that applies to all the press models.
Google and other search engines see these four pages as possible competitors for the search phrase "hydraulic press" and so they are likely to cannibalize each others results and leave the company in a bad over all position for the phrase.
By adding a canonical tag to the individual model pages, which tells the search engine that it should use the category page "Hydraulic presses" as the actual result page, the company dodges this issue and focuses the all the organic traffic that would be assigned to the model pages actually to the product category page "Hydraulic presses".
In practice this means that an HTML tag is added to the HEAD section of the website page template. All modern CMS systems offer the possibility to define a canonical URL so that the marketer does not need to touch the source code. If you have a dev team at your disposal you can ask them how duplicate content and canonicalization is taken into account on your company website. Also, your SEO partner should be very interested in fixing duplicate content issues.