As no one's responded yet, I'll do my best to help you out.
What does it mean when a topic is designated 'hot'? Some topics are really old but still have the 'hot' icon next to them.
This means that a lot of people are still viewing the topic, even if there are no new replies. An example might be a topic called "A Very Young Dancer". Even if the last post was in 2000, people are still searching this on Google, or on here, and accessing the topic for various reasons.
What does it mean when a topic is 'pinned'?
This means that a topic will remain at the top of the forum, regardless of when the last post was, or how the forum is otherwise sorted. They're usually rules.
Why does a board member 'lock' a topic?
A mod might better answer this, but I can think of several reasons based on my own experience. If the topic is a rule, there is no discussing it. Therefore, it will be locked. If the topic is old and has no credence anymore (such as, say, a topic on a performance in 2006), then there is no point in leaving it open. If the topic contains an issue that members feel so strongly about that there is almost a fight going on, then the topic would certainly be locked, if attempts to restore order don't succeed. And if the topic is just plain against the rules, then it will be locked as well.