Red Rain was a genetic weapon that was said to cause the demise of The Elder Race on the planet Cygnus. It's referenced numerous times in Serial 11 "The Planet of Genetic Misadventure" and the audience is able to see this terrible weapon in S11x63 "Red Rain" when Jasper Phillips has a dream that he is a feral cat owned by a human family. Its name sake comes from the Peter Gabriel song also titled "Red Rain".
The weapon is a form of highly corrosive acid rain that dissolves any kind of human or animal in a matter of seconds. This is considered extremely painful and when the attacks happened it was advised to commit suicide as it would be less painful than dying from the acid rain. Even if one were to survive the initial downpour the rain would produce a toxic mist that would kill someone within minutes of breathing it in. It is widely assumed that no humans or animals survived the falling of the Red Rain. Interestingly, we never actually see how this weapon is launched or how the rain forms and falls.
The entire plot surrounding Red Rain seems to be a callback to The Cold War, a real life event where The United States and Soviet Russia were constantly tense with each other and the threat of a Nuclear Apocalypse was something many humans lived with. It can be said that the Red Rain being used could be a Cold War that escalated to Nuclear, or in this case, Genetic Warfare.