HTTPS uses Transport Layer Security, or TLS, to encrypt traffic. More precisely, HTTPS is ordinary HTTP transmitted through a TLS-protected connection, allowing browsers and servers to exchange data confidentially and detect tampering.
TLS also authenticates websites through digital certificates, usually issued by certificate authorities. A browser checks that a certificate matches the requested domain and is trusted before establishing the secure connection. TLS then normally uses public-key cryptography to agree on a temporary session key for efficient symmetric encryption.
A frequent mix-up is SSL, or Secure Sockets Layer. HTTPS originally used SSL, but SSL has been retired because its versions are insecure; modern HTTPS uses TLS, commonly TLS 1.2 or TLS 1.3. HTTPS protects the contents of requests and responses, but it does not hide all metadata, such as the destination IP address or the fact that a connection is taking place.