What is ELK

Elasticsearch is search and analytics engine.
Logstash index the data to Elasticsearch. Logstash has a config file that has input,filter and output section. Config file looks like json file.
Kibana is a visualization tool which provides a web based GUI for a user. User can design the bar, plot reporting charts.
In order to make Kibana and Elasticsearch interact, you need to make both server up and running.
Then logstash will index the data to Elasticsearch. 
Then Kibana would read the data from Elasticsearch and visualize it.