Topics get locked for some time (default is one hour) whenever a user edits, previews or saves a topic. This is to prevent simultaneous topic updates. There is a lock warning if another user tries to edit a topic while the lock is still on.

It is worth noting that all topic changes you do within the lock time will go into the same revision. In other words, if you find a mistake immediately after saving the topic, then TWiki will pretend that it never happened if you fix it within the topic lock time.

You can remove the lock by checking the "Release edit lock" checkbox in preview. This is a way to let other people know that you are done with your editing.