Bumping is when you post in an older thread to make it active again (on this board, that means the subject shows up in bold), so visitors are more likely to check the thread for new postings. It's sort of a way of reminding people about the topic, like if a topic is important to revisit now and then. Another example of a subject that might be "bumped" is if a thread is started to tell people submissions are wanted for the newsletter, and they have to be in by a certain date for the next issue, and that deadline is approaching. Someone might "bump" the thread if it hasn't been active (posted to) in a while.