OTel is a CNCF observability framework (APIs, SDKs, tools) that standardizes producing and shipping telemetry; it was formed by merging OpenTracing and OpenCensus. It is not a time-series database (that is a storage backend like Prometheus), not a dashboard (that is a visualization layer like Grafana), and not a proprietary single-vendor agent (its whole point is vendor neutrality).
Official docs