First of all, we need to depict the question: what is an Elasticsearch index?
The short answer is that an index is like a database in a relational database. Elasticsearch is a document-oriented search and analytics engine. Each record in Elasticsearch is a structured JSON document. In other words, each piece of data that is sent to Elasticsearch for indexing is a JSON document. All fields of the documents are indexed by default, and these indexed fields can be used in a single query. More information about this can be found in the next chapter.
Elasticsearch uses the Apache Lucene library for writing and reading the data from the index. In fact, Apache Lucene is at the heart of Elasticsearch.