Bumping means to put the thread onto the top page.
For example, you create a new thread, which is automatically on the 1st page.
And the newest threads/the threads with the newest posts are on the 1st page.
If you write anything, the thread goes automatically on 1st page.
And some people do this, to prevent forgetting of some threads.
Or there is no reply, because nobody can help you, and you still write something.
Or you write unnecessary things to put the thread on the 1st page.
LOL, no idea how I can describe this better.
Wait for replies, before YOU reply, ok?
Try google for more information!
Greets