The REST API key identifies you account and is the same for all your apps.
It allows you to use Batch API and manage all your use cases: Send push notifications, create custom audiences, attach data to specific users, and more (know more here).
You can find it in your dashboard settings. Go Settings → General:
If you cannot see the REST API key (e.g. Insufficient Privileges error), take a look at this guide.
🚧 Note: The REST API key is sensitive information. You shouldn't share it outside of your company.