The Diffie-Hellman key exchange is a method for two parties to securely establish a shared secret key over an insecure channel. It is based on the difficulty of computing discrete logarithms in a finite field. Each party generates a private key and a public key, and they exchange their public keys. They then use their own private key and the other party's public key to compute a shared secret key. The shared key can be used for subsequent encryption and decryption using symmetric key encryption algorithms. The Diffie-Hellman key exchange is widely used in various applications, including secure communications and online transactions.