What does B and D stand for in personal ads?

B and D stands for "bondage and discipline", which is a variety of sexual activity that involves one person (the "dominant" if male or "dominatrix" if female, or "dom" in either case) physically restraining the other (the "submissive" or "sub"), using handcuffs, ropes, chains, etc. (bondage) and punishing the sub by inflicting mild pain (spanking, for example, but not whipping), forcing the sub to remain in an uncomfortable position for a long period of time, humiliating the sub, etc., especially when the sub breaks the rules that the dom has set.