Definition
A canonical URL is a tag in your page’s code that tells search engines which version of a page is the definitive one. When the same or very similar content exists at multiple web addresses — which happens more often than most people realise — the canonical tag points search engines to the version you want indexed and ranked. It is a way of saying “this is the original; ignore the duplicates.”
Why It Matters
Duplicate content confuses search engines. If the same page is accessible at three different URLs (with and without “www,” with tracking parameters, or through different category paths), Google has to decide which one to rank. It might pick the wrong one, or worse, split ranking signals across all three — weakening each of them. Canonical tags solve this by consolidating authority onto a single URL. Without them, your own site can compete against itself in search results. They are especially important for e-commerce sites where products appear in multiple categories, and for any site that uses URL parameters for filtering, sorting, or campaign tracking.
Example
An online shop sells a jacket that appears under both “/mens/jackets/waterproof-jacket” and “/sale/waterproof-jacket.” Without a canonical tag, Google might index both, splitting link equity between them. By adding a canonical tag on the sale page pointing to the original product URL, the shop ensures all ranking signals consolidate on one page, giving it the strongest possible chance of ranking well.