in the training course, ‘service’ word used with different concepts. My understanding of the word ‘service’ in istio is the Pod or Kubernetes Service. is it correct? and what is workload?
The terms mean the same as they do in regular Kubernetes. A service is something that exposes a workload to the rest of the cluster (or via ingress to the outside world).
A workload is a group of pods performing some work, like a deployment or a stateful set - it is your application you have deployed to the cluster.
Virtual Service which is a much more powerful version of the standard Kubernetes service.