The Canonical Tag, specified as <link rel="canonical" href="[preferred-url]">, is an HTML element that helps webmasters prevent duplicate content issues. It tells search engines which version of a URL is the master or preferred version when multiple URLs exist with identical or very similar content.
Duplicate content can dilute SEO efforts by splitting link equity and confusing search engines about which page to rank. Common scenarios for duplicate content include product pages accessible via different URLs (e.g., with tracking parameters), printer-friendly versions, or pages with slight variations.
By implementing a Canonical Tag, you consolidate ranking signals to a single URL. For example, if https://example.com/product?color=red and https://example.com/product/red both show the same content, you would add <link rel="canonical" href="https://example.com/product/red"> to the <head> section of the ?color=red page. This ensures search engines focus on and rank your chosen URL.