­čů▓ for Canonical

A canonical tag is a piece of code added to a page that denotes the ‘master’ or the main version of a page. It’s pronounced “ka-non-e-kul” by the way, try canonicalisation once you’ve mastered that.

So, what does it do? As Google says in their help doc “If you have a single page accessible by multiple URLs, or different pages with similar content (for example, a page with both a mobile and a desktop version), Google sees these as duplicate versions of the same page. Google will choose one URL as the canonical version and crawl that, and all other URLs will be considered duplicate URLs and crawled less often.” – and this has several benefits.
It helps with crawl efficiency (which is important on bigger sites), it helps show the ‘right’ page in the SERPs (search engine results pages), it helps to consolidate link signals (to this ‘master’ page) and all-round is a good thing to have working for you.
Ecommerce sites especially need canonicals (all sites benefit though) for things like size and colour variants – think of a t-shirt and the different sizes and colours of effectively the same thing/page. The canonical tag consolidates all this to the ‘master’; page. Many sites get this wrong – don’t be one!