More at: http://www.diginabled.com/how-search-engine-crawlers-work/
Webpages are scanned by search engine spiders or crawlers. The terms mean the same. These crawlers find your content by following links. To find your website search engines are reliant on other sites linking to it. Having links from other sites is important for two reasons firstly so that the crawlers can get to your site and second because those links signal the quality and authority of your content. There is no way for search engines to be able to tell if your content is accurate in isolation. They are reliant on signals that indicate authority such signals include links to your site from other sites. Logically sites that have valuable content will also have lots of links pointing to that content.