BTTT means “Back to the Top” or Bump to the top. If you look at the forum/latest comments then the thread jumps back to the top. It’s basically a way of drawing attention to it, saying you agree with it, that kinda thing.
Except that on this site, that ceased to work years ago. So it's a "memory reflex" to old timers. BTTT was a special case of posting "something," the current equivalent is "bookmark."
But that no longer puts the thread at the top of newly posted threads as it used to once upon a time. But it does allow you to find the thread again if you wish to see further posts on a thread of interest, using the "search" function, which still works.
Got it now; thanks.