What does it mean when there is just the word "Bump" listed as a message?

The author is sending (bumping) the message back to the top of the list, so it is not forgotten or lost.

Bringing it back to the top. At least that is what the other forums use it for.... Or BTT (Back to top).

How do you bump a message back to the top?

You just go into the message and type a word, usually BUMP or BTT. Anytime a message has a response it moves it back to the top of the list as an active topic

I'll have to try Bump sometime.

Please don't bump topics.

thank you.

Is this inappropriate for even the classified ads section?

I would say especially for the classifieds section.

Imagine if everyone started bumping their ads. The first page listing and the recent topics would be constantly filled with threads that contain no new information.