Hey Simon,
'Bump' is a term used to describe the "pushing" of a thread back to the top of the thread list by posting in that thread (usually, your own). So, if you have asked for an appraisal, and no one has answered, or no one has answered for a little while, and you want to get some more replies, you simply post within that thread, thus putting your thread back at the top of the list.
The administrators of Namepros have instituted rules against bumping a thread more than once/day, and you may find that if you try to bump repeatedly, you will only edit your previous post & thus, not push your thread to the top of the thread list.
Hope this helps