Canonical URL is the one true URL for a piece of content, like the one ring to rule them all in Lord of the Rings. It's the URL that Google and other search engines should index, and all other URLs should redirect to it like minions bowing before their master.
"Hey Bob, did you make sure to set the canonical URL for that new landing page? I don't want to be up all night dealing with duplicate content issues again."
"Sure, the marketing team can have their fancy vanity URL, but we need to make sure the canonical URL is set to the actual product page. Otherwise, we'll be chasing our tails in the SERP."
What Are Canonical Tags and How Do They Help Your SEO? - Search Engine Journal breaks down canonical tags and their SEO benefits, perfect for when the marketing team starts asking questions.
Canonical URL Tag - The Most Important Advancement in SEO Practices Since Sitemaps - Moz dives deep into the canonical URL tag, for those times when you need to school your coworkers on SEO best practices.
Specify your canonical - Google's own guide on specifying canonical URLs, straight from the source. Trust me, you'll want to bookmark this one.
Note: the Developer Dictionary is in Beta. Please direct feedback to skye@statsig.com.