Canonical tags, or "rel=canonical", are HTML elements used to prevent duplicate content issues in search engine optimization by specifying the "canonical" or preferred version of a web page. It’s a way of telling search engines which version of a similar or duplicate page to index and rank. This is particularly useful for websites that have identical or very similar content across several URLs. By using the canonical tag, website owners can direct search engines to treat a selected page as the original and ignore the others, thus avoiding problems with search engine rankings and user experience.