Smart Search is the search component that comes with Joomla, and this module is its search box. Enter a term and it returns matching content ranked by relevance, with the search terms highlighted in the results.
Smart Search works from an index rather than by scanning the database on every request, which is what makes it fast on a large site and lets it offer suggestions as a visitor types. The index is built once from the Smart Search component in the administrator, and the Content – Smart Search plugin then keeps it current as articles are added and edited. If that plugin is disabled the index has to be rebuilt by hand, and anything published since the last rebuild will not be found.
Because it indexes rather than pattern-matches, Smart Search can also filter results by category, author, type and date, and those filters can be pre-set on a search menu item so a section of the site gets its own scoped search.