bttt means bump to the top.
On the LATEST POSTS page, each thread that has received a response appears at the top of the page, each one that receives a response goes right to the top of the page, and when you hit REFRESH, a NEW thread is at the top.
to BUMP something, means you just responded to a thread, and that thread would appear at the top of the LATEST POSTS page...if you refresh it fast enough!