A "service" is grouping of operations (usually CRUD, but sometimes custom actions as well) on the same REST resource. It is what would be the interface in the SDK (say Java, C# or Python) which contains the methods corresponding to the API operations on the resource.
The service ID is a fully qualified name of interface from the SDK, but in lower case with underscores separating the words. In the latest versions of the vSphere Automation SDK for Java (or Python) there are constants in the interface which represent the service ID, you can see the value of these constants in the javadoc. The constants are named _VAPI_SERVICE_ID and are located in the xxxTypes base interface.
For example
