The Red Room of Pain is a room in Christian Grey's apartment where he practices BDSM in the novel "Fifty Shades of Grey." The room housed all kinds of sexual paraphernalia such as whips, kegel balls, restraints, a St. Andrew's Cross, sex furniture, and chest full of sex toys.
It is important to note that pain in the context of BDSM is used not to inflict punishment, but to increase arousal and sexual pleasure.
More About Red Room of Pain
The Red Room of Pain is where Christian Grey introduces the novel's female protagonist, Anastasia Steele, to the tools and toys of BDSM. After the "Fifty Shades" trilogy became a major bestseller, people started making Red Rooms of their own in their homes - or at least shopping for enough toys for a Red Drawer of Pain.