Canonical url - what is it?

It's how you tell Google which version of a page is the real one when the same content is reachable at several URLs, like with and without tracking parameters, or a product sitting in two categories. You add a link rel=canonical tag in the head pointing at the main URL. Two things people get wrong: it's a hint, not an order, so Google can pick a different one if your signals disagree (internal links, sitemap). And every normal page should canonical to itself. Pointing all your pages at the homepage is a classic mistake that drops them out of the index.