Threads are displayed on the main forum page in the order of the thread's last post's date and time.
Whenever anyone responds to a thread with a new reply, that thread gets a new date/time based on the new posting, and so goes to the top of the list. At least until some other thread (either brand new, or responded to by another poster) which has a more recent date/time takes it's place at the top, and all threads move down the list one position.
When a thread is 'bumped', it means someone (either the OP or another user) has responded to the thread, not with additional comment, but merely with the word 'bumping', or something similar, indicating that their intent is to move the thread to the top of the display, and not necessarily to add any additional discussion. This is generally a case where due to high board activity a thread has slid down the page pretty far, and someone 'bumps' it so that others who may have missed it, may see it.
The
DVC Rent/Trade board is a special case. On that board, 'bumping' by the OP is a requirement of the R/T board posting rules. To help keep that board running smoothly, the OP of a thread must maintain it, and cannot just post and then forget about it. They must bump it at least weekly, thus showing that they wish for the thread to remain active, meaning their 'request' or 'offer' is still available. When they no longer need the thread, they must edit the title to say 'Closed'.
On the R/T board, threads not maintained by the OP as required above, are locked by the Moderators.
This is why on the R/T board you'll see so many threads that have 'bumping' in them.
And just to answer another question before it is asked:
OP = "Original Poster", that is, the thread originator.