There is a how-to page (Search Sites) that explains what the search, in the bottom bar, searches.
The important thing to note, search works within the browser using the content of the sitemap file that get loaded when a site is added to your current neighbourhood.
The sitemap contains an entry of each page on the site containing: * slug - the name of the file containing the page, * title - the page title, * date - the number of milliseconds since 00:00:00 (UTC), Thursday, 1 January 1970 * sysnopsis - the first paragraph from the page See example, for this page, below.
{"slug":"what-does-search-search", "title":"What does search search?", "date":1418981882591, "synopsis":"There is a how-to page (Search Sites) that explains what the search, in the bottom bar, searches."}
Search looks at the title and first paragraph of all pages in your Neighborhood. As you browse the federation more sites are added to your neighborhood so search can find more pages.