Symmetric cryptography, also known as secret-key cryptography, uses a single secret key to both encrypt and decrypt messages. This key is a unique and private piece of information that must be kept confidential to ensure the security of the communication. It is used in various encryption methods, such as in protecting user information in 1Password and authenticator apps for secure device access. In summary, a secret key is a vital component in symmetric cryptography and is crucial for maintaining the confidentiality and security of sensitive information.