Mm... Supposedly, its the abbreviation of "Bondage and Discipline, Dominance and Submission, Sadism and Masochism."
Umm... If you want to know... Its feeling sexual pleasure by bonadge... Like being tied tightly, being blindfolded... use of physical restraints, the granting and relinquishing of control, and the infliction of pain. That's basically the gist of it.