Pillow links are the term used to describe the backlinks that are not a part of your link building strategies, with a purpose to diversify your overall backlink profile.
Google doesn’t like people to do SEO or build links as a method to improve search engine ranking. So if your backlink profile doesn’t look natural enough, such as if the majority of your backlinks are do-follow or if they are mostly text-links, then that could raise a flag and put your site at risk for a penalty.
Pillow links, therefore, are used to make your backlink profile look natural, by adding different diverse links to your backlink profile. For example, no-follow links, image links, local citations, etc.