What role does Grafana play next to Prometheus?

Grafana is the visualisation layer. It connects to Prometheus as a data source and draws dashboards whose panels are each backed by a PromQL query.

Grafana is not part of Prometheus, but it has become the standard tool for visualising almost any kind of data. The Prometheus developers first planned their own GUI, PromDash, then dropped it because Grafana already did everything needed and had far more maintainers. The Prometheus data source in Grafana is now maintained by the Prometheus developers themselves.

What makes it practical:

• Each panel = one query. You enter a PromQL expression and Grafana handles the rendering.
• Ready-made dashboards on grafana.com, e.g. for Node Exporter or MySQL, can be imported by ID.
• Composing dashboards from parts of others, for a wall screen in the operations room.
• Several data sources at once: many Prometheus instances, or InfluxDB for long-term data, side by side.

Grafana listens on port 3000 by default.

Go deeper:

• Prometheus — Grafana support for Prometheus — the official how-to for the data source and a first graph.
• Grafana — Wikipedia — what Grafana is and which data sources it supports.