In short. Prometheus is a worker that knows about all your other services. Each service of interest can expose an endpoint that prometheus scrapes periodically. So the services just say what the current state is, and prometheus, since it keeps asking, knows and stores what happens over time. Grafana is a web service that uses prometheus as a data source and can visualize it very nicely.

Prometheus also comes with an Alert Manager, where you can set up rules for when to trigger an alert, that can end up as an email or slack integration.

They are all very useful, and gives a much needed insight into how things are going.