What it's for
Port 5601 is the default port for Kibana, the visualization and dashboarding layer of the "ELK stack," built to query and display data stored in Elasticsearch. Rather than writing raw Elasticsearch queries against port 9200 directly, teams use Kibana's web UI to build dashboards, explore logs, and create visualizations on top of that same underlying data.
Protocol basics
Kibana serves a standard HTTP(S) web application on port 5601, which itself communicates with Elasticsearch (typically on port 9200) behind the scenes to fetch the data it visualizes. Since Kibana is essentially a window into whatever data lives in Elasticsearch — including potentially sensitive logs or application data — it should carry the same authentication requirements as the underlying Elasticsearch cluster, and shouldn't be exposed to the public internet without access controls in place.