Nofollow links are internet links that don’t pass on SEO ranking.
From a user perspective everything looks the same, but from a search engine perspective it tells search engines not to pass SEO “juice” to the linked site
Why bother using nofollow?
It’s very useful for letting search engines, like Google, know that content is sponsored content (example: ads).
That way you can still have ads and not be penalized for running link schemes, boosting SEO for other other sites in exchange for money.
It’s also very useful for sites that accept content from users, such as blog comments and forum posts. That way spammers don’t target your site for SEO “juice”.
Therefore, it keeps the site quality high, which keeps Google happy.