In some conversations on message boards, it is used in place of the word "random" to indicate that a post is off topic.
It may also be used to indicate that the content of a post is inappropriate for that board should be posted in /b/ instead.
This is due to the fact that "/b/" originates from the 4chan
"random" board. the /b/ board is full of all manner of obscene madness that you wouldn't even see in sparta.