Key exchange allows two parties to establish a shared secret over an insecure channel — without ever transmitting the secret. Diffie-Hellman (DH) and ECDH are the mechanisms.
DH solves the symmetric key distribution problem. Ephemeral DH (DHE/ECDHE) generates new keys per session for Perfect Forward Secrecy. Used in TLS handshake. Quantum computers threaten DH/ECDH — post-quantum alternatives (CRYSTALS-Kyber) are being standardized.