Glossary
Canonical tag
A canonical tag tells search engines which address is the real one when the same content is reachable at more than one URL.
Duplicates happen far more easily than people expect: with and without www, with tracking parameters attached, with and without a trailing slash. Each looks like a separate page to a crawler.
Why it matters to you
Without one, a page can end up competing with itself, splitting its signals across several addresses and ranking as well as none of them.
An example
A product reachable at four URLs because of tracking parameters is four pages as far as Google is concerned. One canonical tag makes it one page again.
Where this comes up: search engine optimisation .