Google exists to serve the searcher. This means that Google is constantly trying to find out which results are most relevant for a particular searcher and for a specific time. In addition to relevance, Google also takes credibility into account. The search engine checks whether other websites are linked to yours. If so, it means that your content is worth a link and is therefore more credible than a website that is not referred to externally. In short, link building is what SEO professionals do to get links to their websites to improve search results.