When Gandalf yelled at the Balrog and broke the Bridge of Khazad-dûm, it's just like invoking the
rel="nofollow" attribute that can be summoned for links on the internet.
It looks like...
<a href="https://definitelynotaspamlink.com" rel="nofollow">Grow your audience by 10000% with this cool new trick</a>
nofollow is useful to prevent Spamrogs from posting links on your valuable content that then get followed and indexed by search engines, causing SEO sadness.
You want to use this attribute on any user-generated content (UGC) that doesn't pass an editorial review. Think comment section. A lot of tools do this automatically for you (but it's good to double-check).
For example, on GitHub (which has a lot of UGC), links automatically get the
nofollow value. Right-click any link in an issue and Inspect to see!
Have a lovely day,