What is meant by topic locked and why was it locked.

"Topic is locked" means that a moderator or Admin has locked the topic so nobody can post any more replies to it. This is done for various reasons. Some topics have pretty much "run their course", some have gotten heated and need to be locked to prevent flaming, and some have gone so far off the original topic that they are all but impossible for the moderators to sort out and bring the discussion back to the original topic.

There are other reasons why a topic may be locked, but these are the ones that leap to mind.