Link building refers to the marketing efforts to get links from other websites to your website. It’s seen as one of the most powerful tools to achieve higher rankings for your site in search engines. If a lot of high-quality links lead to a certain page, search engines will consider it a popular or meaningful article, and, therefore, they’ll rank it higher.
Link building is when you get other websites or blogs to link to your site's page in order to improve your search engine rankings. The engines crawl the web looking for links between your web pages and other websites to decide how valid your content is and thus where your page should rank in their search results.
The exact reason why SEO is making money. But its just a hack and no long term solution towards proper marketing.