Link building is gaining links from other sites pointing to your own, in order to raise its position in search. It is one of the pillars of SEO, alongside the content itself and the technical side of the site. What counts is the quality and naturalness of the sources: valuable links build position, while those bought in bulk risk a penalty from Google, which is why the emphasis today is on links won through content and relationships. (Compare: Backlink, White Hat SEO)