Elasticsearch is an open-source search and analytics engine that allows for searching both structured and unstructured data in (near) real-time. The document discusses how Elasticsearch uses Lucene's inverted index architecture under the hood and can be used as a plug-and-play replacement for other search engines. It then provides examples of how the company uses Elasticsearch for centralized logging, log monitoring, network monitoring, and generating comparison reports by modeling data as graphs in Elasticsearch.