Apache Solr is an open-source search platform built on the Apache Lucene library, designed to provide efficient search and retrieval of data for web applications, especially those with large datasets. It offers advanced features like faceted search, customizable indexing, and relevance scoring, making it suitable for robust applications such as content management systems and enterprise search. The document outlines the integration of Solr with Drupal to enhance the search experience, providing a seamless and user-friendly interface for users navigating large quantities of information.