Like Drupal 7, Drupal 8 core comes packaged with its own built-in search module. This provides usable search functionality by querying the Drupal database directly. The Drupal core search allows you to search for user profiles and for node content. It periodically queries the database to maintain an index of content without requiring any prior configuration.
The Drupal search may be fine for your needs if you only require basic search functionality and you don't have a large amount of site traffic. However, it has several functional limitations which include the following:
It uses the Drupal database to carry out its queries, placing extra load on the database which may already be a performance bottleneck on your site
It's not possible to control which nodes are indexed for the search
The content is always indexed based on the default display mode of each node
The search only matches whole keywords for nodes
If you mind about any of these limitations, you should switch to...