For more information see the Authentication and Client Secrets Guide.
The Authentication and Client Secrets guide contains details on how to use this endpoint. However, for clarity, the client ID is simply a string that uniquely identifies your application, and can be any string up to 50 characters long. If you have, for example, a website and a background task that checks for details offline, use two separate client IDs (for example: MyOrganisation-Website and MyOrganisation-OfflineUpdater). The client ID is used by our support teams if you have a problem.