Red Shirt is usually a person who exists just so that they can be killed off in the Story. Red Shirts are commonly unnamed, but not always, and will die in a spectacular variety of ways. The term derives from the pop culture red shirt meme that was introduced through the original Star Trek franchise where random crewmembers would die while wearing red coloured uniforms.
During Space Camelot, events that took place around 500AD, some of the Knights of the Round Table would wear red tabards, indicating to the Reader that they were bound for imminent death. These knights were often the first on 'Away Missions' and their deaths would serve the survival of later Characters who venture in their footsteps[Pan 1].
A small group of three redshirts show up dead having been melted by the sulphuric acid rain on the planet Venus. This prompted the use of spacesuits found aboard Camelot by the second away team led by Sir Lancelot du Lac[Pan 1].