From official website, Elasticsearch is a distributed, RESTful search and analytics engine capable of addressing a growing number of use cases. As the heart of the Elastic Stack, it centrally stores your data for lightning fast search, fine‑tuned relevancy, and powerful analytics that scale with ease.
Data in Elasticsearch is organized into indices. Each index is made up of one or more shards. Each shard is an instance of a Lucene index, which you can think of as a self-contained search engine that indexes and handles queries for a subset of the data in an Elasticsearch cluster. For details on shards and indices, see here