RACK is the abbriviation for Risk-Aware, Consensual Kink. It was first introduced by Gary Switch as an improvement over SSC (Safe Sane Consensual). His statement can be found here. The argument is that no BDSM activity is ever safe. Any torture always bears a risk. Sane is also an issue because it is very difficult to define.
Critics of the term RACK counter that risk-aware is as hard to do and define as sane. Are you aware of the risk of your partner having a heart condition? In addition to that, many people feel that it is just another label for the same idea and Gary Switch is simply being overly critical of a few simple words.