dd-trace is an npm package that you can install in your Node.js application to capture APM (Application Performance Monitoring) data. In Datadog terminology this library is called a Tracer. This data is then sent off to a process that collects and aggregates the data, called an Agent. Finally, the data is sent off to the Datadog servers where it's stored and made available for querying in a myriad of ways, such as displaying in a dashboard or triggering alerts.