Prometheus is fundamentally a metrics based monitoring system, with a time series database (TSDB) at its core. It is fully open source.
The data model features labels, arbitrary key-value pairs to identify your metrics.
It features a powerful language called PromQL that can use these labels, and which is used in graphing and alerting.
There's a broad ecosystem, building on the extensibility of Prometheus. This includes client libraries to instrument your own code, exporters to integrate with existing instrumentation and APIs, graphing tools, service discovery to learn about targets from EC2/Consul/Kubernetes/etc. and notification integrations to Email/PagerDuty/Slack/etc.
A key aspect of Prometheus is a desire for reliability and thus simplicity.
Let's dig a bit more into these aspects of Prometheus.