How does the log get into Kibana? What is Elasticsearch?

The log was ingested into Kibana through a chain: the application generates logs, which are collected and sent by a log agent (such as Filebeat or Logstash) to Elasticsearch — a distributed storage and search engine. Kibana is a web interface for visualizing and analyzing data stored in Elasticsearch.

Elasticsearch is a system based on Apache Lucene, designed for full-text search and real-time analytics. It stores data as JSON documents, supports scaling, and provides fast search over large volumes of data. In conjunction with Kibana and Beats/Logstash, it forms the Elastic Stack (formerly ELK Stack) for collecting, storing, searching, and visualizing logs and metrics.