There are 6 buttons at the bottom left page of every topic in every forum you moderate. They are left to right as follows:
Delete, Garbage, Move, Lock, Split, Merge
The delete button removes the topic.
The garbage button moves the topic to the trash can(if there is one).
The move button allows you to move the topic to another forum.
The lock button prevents all users from posting in the topic unless a mod unlocks it. Mods and admins can post in locked topics. But, for example. let's say there is a locked topic in the Hello/Goodbye forum. You are the only mod of that forum. Only you and the admins can post in that locked topic. If you are not a mod of a forum, you cannot post in a locked topic in that forum.
The split button removes posts from topics, and moves the post to another topic.
The merge button moves a topic into another.