We're used to seeing certain descriptors repeat in the ESRB ratings: Strong Language, Blood and Gore, that sort of thing. Some bit of content in Metal Gear Solid 5: Ground Zeroes has earned it one of the rarest descriptors that the ESRB gives: "Sexual Violence."
Kotaku noticed the ratings descriptor and contacted the ESRB, which said that only 2006's Animamundi received the same content warning. The ratings summary includes some more information on why it earned the rating: "The game includes an audio file in which a female character is sexually assaulted by male characters; while there is no visual depiction, sounds of ripped clothing and struggle can be heard."
Some recent games have had some level of overtones, but mostly as an implied threat. An actual sexual assault, even strictly in audio form, is taking it much further than most games. If nothing else the descriptor can serve as a trigger warning to real-life survivors who may be interested in playing the game.