A canonical page is a preferred version of a web page that search engines should consider as the main source of content. This is important when multiple pages have similar or duplicate content, as it helps to avoid confusion and ensures that the correct page ranks higher in search results. Webmasters use a special HTML tag called the rel="canonical" link element to indicate the canonical page. By doing this, they guide search engines like Google to understand which version of the page to index and display, improving the site's overall SEO performance.