A client secret is a confidential key used in authentication processes, particularly in applications that interact with APIs. It helps verify the identity of a client application when it requests access to resources. This secret is typically generated when the application is registered with an authorization server, such as those used in the OAuth 2.0 framework. The client secret should be kept secure and not exposed in public code repositories or client-side applications. If compromised, it can allow unauthorized access to sensitive data or services. Proper management of the client secret is crucial for maintaining the security of applications and their users.