NodeBB comes bundled with the "dbsearch" engine, which does basic fulltext search using the database itself. Other search engines are available for download as third-party plugins, for Solr and Elasticsearch.
You can find these plugins available for install in the "Extend -> Plugins" page. In nearly all cases, after installation, activation, and a NodeBB restart, you will need to enter the plugin's configuration page to enter some configuration info before searching will work. If you have existing content already, you may want to clear the search index and re-index the content.