Smart Search is the search component built into Joomla. It builds an index of your site's content ahead of time, so results come back quickly and can be filtered, rather than running a slow query across the database on every search.
Smart Search covers articles, categories, contacts, news feeds and tags out of the box, and other extensions can add their own content to the index. Presenting it on the front end takes two pieces: a Smart Search menu item for the results page, and the Smart Search module if you want a search box in a template position.
Before search will return anything, the index has to be built once from Components → Smart Search in the site administrator. After that it stays up to date automatically as content is saved, and it can also be re-indexed on a schedule. Smart Search documentation