When you are talking to a friend on the computer and you both run out of things to say. If you say TBT you are excused and dont have to say anything until you think of something interesting to talk about, provided the other person says TBT back.
Magi: Yea... TBT?
(20 min later)
Magi: Guess what? I got a new dog :D
Gio: Noway! Thats so cool :P