OAuth is a system that allows you to grant a third-party application limited access to your information on another service, without sharing your password.

Here's a simplified breakdown:

1. You want to use App A (e.g., a photo printing service) with your photos from Service B (e.g., Google Photos).

2. App A asks for your permission to access your photos on Service B. It redirects you to Service B's website.

3. On Service B's website, you log in (if you aren't already) and see a clear request from App A, detailing what it wants to access (e.g., "view your photos," "manage your albums").

4. You approve this request. Service B then gives App A a special, temporary "access token."

5. App A uses this access token to interact with Service B on your behalf, but only for the permissions you granted. It never sees your Service B password.

6. If you revoke access later (usually through Service B's settings), the access token becomes invalid, and App A can no longer access your data.