A canonical tag (<link rel="canonical" href="...">) lives in the <head> of your HTML and tells search engines: "this is the definitive URL for this content." It's essential when the same content is reachable from multiple URLs — which happens more often than most teams realize.
The classic landing page scenario: you run paid campaigns with UTM parameters. Without a canonical tag, Google might index /pricing?utm_source=google&utm_medium=cpc as a separate page from /pricing. Now you have two URLs competing for the same keyword. I've seen sites with dozens of indexed UTM variants diluting their authority across pages that are literally identical.
Common canonical mistakes on landing pages
The worst mistake is a self-referencing canonical that points to the wrong URL — like HTTP instead of HTTPS, or www instead of non-www. This silently splits your ranking signals. The second most common: A/B testing tools that create alternate URLs without proper canonicalization. Every variant page should canonical back to the original unless you're intentionally testing SEO variants.
Quick check: view source on your landing page, search for "canonical." If it's missing or points to a different page than the one you're on, fix it immediately. This is one of those five-minute fixes that can quietly double your organic traffic to a page.