Heartbeat executes periodic checks to verify whether the endpoint is up or down, then reports this information, along with other useful metrics, to Elasticsearch. In this case, you've asked Heartbeat to check the uptime for the availability of the website https://www.elastic.co. Heartbeat then sends this data to Elasticsearch and you can see the data in the Uptime app.
Heartbeat was set to use the default ports for Elasticsearch and Kibana in this example. See the documentation for configuration options.