Locking means it changes the password to a value that nothing could ever hash to (by putting a ! in front of it, for example). Unlocking just removes the !. Th

Locking means it changes the password to a value that nothing could ever hash to (by putting a ! in front of it, for example). Unlocking just removes the !. The security disadvantage is that if someone gets your /etc/shadow file they can still see the password hash, in case they wanted to try to crack it, but honestly at that point your server is likely compromised anyway.