Most links tell Google "follow this link and give it SEO credit." A nofollow link says the opposite — "users can click this, but don't count it for rankings." It's an HTML attribute that controls how search engines treat outbound links from your site.
Nofollow links are commonly used in blog comments, sponsored content, and user-generated content to prevent spam. If someone drops a link in your comment section, the nofollow tag ensures it doesn't boost their SEO artificially. Users can still click and visit the page — it just won't pass any "link juice" for ranking purposes. It's an essential tool for maintaining clean SEO practices.