Qualify your outbound links to Google

For certain links on your site, you might want to tell Google your relationship with the linked page. In order to do that, use one of the following rel attribute values in the <a> tag.

For regular links that you expect Google to follow without any qualifications, you don't need to add a rel attribute. For example:

<p>My favorite horse is the <a href="https://horses.example.com/Palomino">palomino</a>.</p>

For other links, use one or more of the following values:

rel values
rel="sponsored"	Mark links that are advertisements or paid placements (commonly called paid links) with the sponsored value. More information on Google's stance on paid links. <a rel="sponsored" href="https://cheese.example.com/Appenzeller_cheese">Appenzeller</a> Note: The nofollow attribute was previously recommended for these types of links and is still an acceptable way to flag them, though sponsored is preferred.
rel="ugc"	We recommend marking user-generated content (UGC) links, such as comments and forum posts, with the ugc value. <a rel="ugc" href="https://cheese.example.com/Appenzeller_cheese">Appenzeller</a> If you want to recognize and reward trustworthy contributors, you might remove this attribute from links posted by members or users who have consistently made high-quality contributions over time. Read more about avoiding comment spam.
rel="nofollow"	Use the nofollow value when other values don't apply, and you'd rather Google not associate your site with, or crawl the linked page from, your site. For links within your own site, use the robots.txt disallow rule. <a rel="nofollow" href="https://cheese.example.com/Appenzeller_cheese">Appenzeller</a>
Multiple values	You may specify multiple rel values as a space- or comma-separated list. Examples: <p>I love <a rel="ugc nofollow" href="https://cheese.example.com/Appenzeller_cheese">Appenzeller</a> cheese.</p> <p>I hate <a rel="ugc,nofollow" href="https://cheese.example.com/blue_cheese">Blue</a> cheese.</p>

Links marked with these rel attributes will generally not be followed. Remember that the linked pages may be found through other means, such as sitemaps or links from other sites, and thus they may still be crawled. These rel attributes are used only in <a> tags (because Google can follow only links pointed to by an <a> tag), except nofollow, which is also available as robots meta tag.

If you need to prevent Google from following a link to a page on your own site, use the robots.txt disallow rule.

To prevent Google from indexing a page, allow crawling and use the noindex robots rule.