A canonical url is the preferred url
It is a way of telling search engines like Google that you have multiple pages with the same content, and with the use of the canonical tag you let them know what is the most important/best page.
Let’s say you have a website about various kinds of drinks. One thing you can drink is sparkling water. There are several brands that make and sell sparkling water, so pages about these will look about the same. With the canonical tag you will let Google know which page should be considered as your primary page. On the duplicate pages you will put the canonical tag and have the primary page mentioned there.
If you want to know more about how to use it, you could read this canonical manual.
Image source: Smartranking