Definition:
StatsD is originally an easy daemon developed and released by Etsy to aggregate and summarize application metrics. With StatsD, applications are to be instrumented by developers using language-specific client libraries. These libraries can then communicate with the StatsD daemon using its dead-simple protocol, and therefore the daemon will then generate aggregate metrics and relay them to virtually any graphing or monitoring backend.
Further Reading:
Book: The DevOps Handbook by Gene Kim, Jez Humble,Patrick Debois.