A locked (closed) padlock used to secure a latch or chain, or as an icon of a secure Internet connection, private account, or locked device.
Often depicted with a gold base and silver chain.
Often used in content about safety, security, privacy, and protection, and various locked feelings, such as locked/down.
This emoji does not appear in page titles on Apple's Safari web browser, presumably to prevent lower-profile sites from pretending to have HTTPS installed for encryption.