This read-write enumerated INTEGER (locked/unlocked) sets whether an interface's MAC-learning port-security feature is administratively locked, and per the description, unlocked ports learn all MACs and forward all frames while locked ports stop learning new MACs and only forward known sources. Admins use this to secure sensitive access ports against unauthorized devices being plugged in, since locking prevents new, unrecognized MAC addresses from gaining network access. For example, locking a port after the authorized workstation's MAC has been learned prevents a rogue device plugged into that same jack from communicating.