A red 2 message was an important transmission sent out by Starfleet Command to their vessels.
In 2267, after the universe had experienced a mysterious magnetic effect that for a split-second caused it to be "winked out" of existence through the actions of Lazarus. Starfleet Command sent the USS Enterprise a red 2 message along with a Code One Alert from Commodore Barstow, which Captain James T. Kirk had Lieutenant Uhura verify. The message signaled to Kirk the issuance of a Code Factor 1: that an invasion was imminent. (TOS: "The Alternative Factor")