This specifies the cloud object to download from Cloud Storage. You can view these by visiting the "Cloud Storage" section of the Cloud Console for your project.
A single asterisk can be specified in the object path (not the bucket name), past the last "/". The asterisk behaves consistently with gsutil. For example, gs://my-bucket-name/pre/a_*.txt would match the objects in cloud bucket my-bucket-name that are named pre/a_2.txt or pre/a_abc23-4.txt, but not pre/a_2/log.txt.