One of the many factors that search engines look at when determining your site's relevance and importance amongst the others that pertain to a search parameter is link analysis: how many other sites link to yours. As mentioned elsewhere in this SEO guide, site popularity equates to many search engines as site importance.
Generating Links to Your Site
The first and best way to encourage people to link to your site is to have up-to-date, interesting content. Every time a website or blog, or Facebook post points to your page, it adds to your site's "popularity." And search engines take notice.