ASP.NET Machine Keys are cryptographic keys used by ASP.NET applications to secure sensitive data, such as authentication tokens and session information. These keys ensure that data exchanged between the server and clients is encrypted and protected from tampering or unauthorized access. The machineKey element in the web.config file allows developers to specify custom validation and decryption keys, enhancing security by preventing attackers from forging authentication cookies or accessing session data. Proper configuration of machine keys is crucial, especially in web farm or load-balanced environments, to maintain consistent encryption and decryption across multiple servers.