When you add an application you are provided with a client ID and client secret for the application. You must supply the client ID when you call an API that requires you to identify your application by using a client ID, or a client ID and client secret.

To register an application click on Apps in the main menu and then click on the 'Register an application' link. Once you have provided an application name, description, etc you will be shown your application client ID and client secret.

Make a note of your client secret because it is only displayed once. You must supply the client secret when you call an API that requires you to identify your application by using a Client ID and Client secret.