'Redshirt(s)' is a slang term for a fictional character/characters in a film/book/tv show who will obviously be killed off quickly and without undue fuss in order to further the plot. Examples include various horror fiction stereotypes, as well as policemen, dumb blondes and other interchangeable figures in other fiction
The term was first used by fans of the sci-fi show Star Trek, in which the protagonist security forces wore red uniforms. This security force maintained an extrordinarily high on-screen mortality rate; the death of a redshirt or two being the writer's plot device of choice to indicate danger or raise dramatic tension. The term has since risen to a generic status, and may be used concerning any form of fiction.
They became rare in later series because the security forces started wearing yellow and they had much better writers. The coolest redshirt death was in the new film; we got a couple bits of info out of him (he was from England and hated Romulans) and he died because he was being cocky.