This is the "{project name}-xxxxxxxxxxxx.json" file associated with the service account credential in the Google Developer Console. A new private key may be generated for a service account if you no longer have the original.

To create a new service account:

To regenerate the client secrets file for an existing service account, follow the above instructions until the "Create new Client ID" step and instead find the Service Account for which you need a new key and click "Generate new JSON key".