Specific forums on image boards are often identified via a single letter code like this. What /b/ stands for...frankly, I don't know, and that's perfectly fine by me.
/b/ stands for random. Having the letter b itself was a random choice to represent random. Its the part of the image board where virtually anything goes and is accepted.
You're not supposed to talk about /b/ because they consider the internet to be very serious business sometimes, and keeping themselves 'anonymous' is part of that. If people don't even know what /b/ is how will they know who anon is (besides being an internet hate machine)?
http://en.wikipedia.org/wiki//b/#.2Fb.2F
"meaning /b/'s posting rate had attained an average of more than 111,000 posts per day."