ETW stands for Event Tracking for Windows .So, what is ETW, simply put, ETW tracking means you can see tracking information in the famous Event Viewer.
How to do it
Let us create an extremely simple workflow just for tracking.
Workflow: Workflow1.xaml

Enable ETW tracking:
Open Event Viewer, Navigate to Event Viewer->Applications and Services Logs->Microsoft->Windows->Application Server-Applications. Right-click Application Server-Applications and select View->Show Analytic and Debug Logs. After refreshing the node, you should see:

Host: Program.cs

Result:
After running the workflow, you right-click the Analytic node, and select refresh, then you should see:

How it works
Here is the WF4 Tracking architecture:

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WF4