Elasticsearch is a full text search engine. It has 2 main purposes:
The first is to be able to search its content according to a query and retrieve the corresponding documents.
The second is to sort these documents according to their relevance.
To do so, Elasticsearch computes a score according to the request.
This score is influenced by each part of the query but the most sophisticated feature is its ability to tokenize words in a text field and ponderate it according to the frequency of these words in the corpus.
You can find more information about scoring in the Elasticsearch documentation.