I'm sorry if this question sound funny, I have always noticed a symbol of padlock on some posts made, and this is irrespective of rank,I use to think the post is locked from reach,like no reply is to be made into the post, or maybe the post is outdated (has stayed long enough), please, I care to be sure.
There is your answer. Not that hard to guess. The meaning is in the word itself. And I don't know if there's another meaning that a padlock suggests.
If I may add, though. If I'm not mistaken, a post cannot be locked; only a whole thread can be locked. And a thread could be locked either by the OP or by a moderator. If it is by the OP, it could be unlocked and posts and replies are opened once again but if it is locked by a mod, the OP cannot open it anymore.