These little robots go to your site and "read it" in similar way as a human does, left to right, leading to base. What these robots discover they remind the "mothership," the search engine data source. If Googlebot finds your site about kittycats, it will report the site, its area (URL), its contents, and so on to Google's database where it is stored. When someone then does a search with Google for kittens, Google seeks to its own data source for search engine result. This is just what makes search engines so quick. All the searches you do call upon information already in the online search engine's database, not from a brand name brand-new search of the whole net.