In this video you will see what are Applications in SAP Customer Data Cloud and how to create and manage applications.

Application Keys are credentials given to third-party applications to enable them to access the Customer Data Cloud platform and make system calls. An application key is, in fact, a special type of user key, which is not associated with a specific user. When you add a new application, you create a user key without requiring a user registration flow. Application keys have higher rate limits than standard user keys, but their actions are not audited. Most API requests to CDC should be made securely, using an authentication mechanism. The recommended method is to use a bearer token. The other option is to use an application key and secret. 

Signing Requests to SAP Customer Data Cloud:
https://developers.gigya.com/display/GD/Signing+Requests+to+SAP+Customer+Data+Cloud