What does bump a thread mean?

[BooLoo2012]

So I have read the Global Forum Rules about a million times now, and I still don't know what "Do not bump a thread that has been cold for over two weeks." means. What does "Do not bump a thread" even mean? Plez halp

 

[GreenUprooter]

To bump a thread usually means to post a random message on it which will make it pop up in a recent or unread messages page, so that people will see it even when it's gotten shoved behind other threads.