Means: Tell Me When You're Back. Used specially in internet language, it's used when two people are chatting on the internet and one of them leaves for a while, the other asks to know when his/her friend will be online again.
Note: The final B may be changed and use a "R" (to mean ready) for non-internet uses
A: Dude, that was awesome!1
B: Yeah!! Hey, dude, I gotta go, I'm gonna be afk for a while
A: OK, then TMWYB then, ok?
B: Sure, dude!!