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.
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.