This is not groovy per se, but the testing framework called Spock (which is very popular among Groovy developers, for good reasons 🙂 – http://spockframework.github.io/spock/docs/1.0/index.html
This expression in particular is a way to instruct Spock that it should expect exactly one call to the method prova in myService, and that this call should be mocked to return true. See Interaction based testing , in particular the section called Combining Mocking and Stubbing.