Bondage, Discipline & Domination, Submission & Sadism, Masochism. (The "&"s are where the same letter is shared, not any linking of the two concepts.)
It's a generic name for all that stuff where something resembling power is exchanged in the context of a sexual relationship, in much the same way as LGBT(*) is a generic term for sexual relationships where gender/sex norms are unusual.
Within the BDSM communities, you'll find they usually use the letters "SSC", which stands for Safe, Sane, & Consensual - essentially do what you want with one another, but make sure everyone consents and that lines of communication remain open so if consent is withdrawn it can be communicated, practice safety at all times (it's relatively easy to accidentally injure or even kill someone if you restrain them, for example), and, well, snuff scenes are probably not sane.
Contrary to the grandparent's assertion, there's no opposition to BDSM from the majority of people interested in social justice - in fact, attempting to suppress someone else's sexuality is generally frowned upon by social justice types.