If you have a function which invokes a different object (or more commonly, a different network service), you pass that as a parameter instead of making a call inside the function.

foo() { service = someRemoteService(authn, authz); http://lookingglass.internal.adm.bol.com/?page=TAM+Middlewar... }

is replaced by

foo(service) { do_something_with_service(service) }

It makes replacing service with fake_service_object easier, thus simplifying the test process.

Common use case: If your service was a remote database, you can replace the database handle with a mock object which pretends to be a DB but is actually a simple object returning hardcoded values.