If you mean bump, it basically means "bring up my post" Auction hosts are allowed to do this 3 hours or more after the most recent post to bring their post back into the recent threads list.
There is a list on the homepage of this website that says recently active threads. It has threads everyone sees that people just posted on. When you "bump" you put your auction on that list in hopes others will see your auction thread and bid a higher price.
You can only bump an auction to put your auction on the recent thread list once every 3 hours so spamming is avoided.