Redshirts is half Star Trek parody and half metacommentary on the nature of reality. The premise is that the protagonists are ensigns onboard the starship Intrepid, which is a transparent stand-in for the starship Enterprise that the narrative even acknowledges in the most hilarious way possible. There's something very strange going onboard the Intrepid.
The number of deaths on away teams are horrific and far in excess of what would normally happen. Its ensigns just make stupid errors left and right, often at dramatically appropriate moments. Also, the science onboard the ship is nonsensical as well. The dedicated geniuses onboard don't understand why "The Box" is able to do half the things it does. Finally, there's a deranged scientist living in the ship's Jeffrey's tubes (sorry, cargo ducts), that know's what's going on but has been driven mad by the realization.
The four protagonists are a bit interchangeable with the exception of Dahl and Duvalle. Dahl is a former alien seminary student and Duvalle is the only woman in the group. They're all incredibly snarky, irreverant, and very entertaining but could have used a bit more differentiation. Part of that is the joke, though, that they exist to be interchangeable parts on a ship that values their lives less than a french fry in a happy meal.
This is a book that benefits significantly from your knowledge of Star Trek, especially The Original Series. In fact, I'd argue that the joke for Redshirts doesn't really apply to subsequent series of Star Trek. The "redshirts are disposable" joke started in the Sixties and was so ubiquitous that they actually made two episodes of Star Trek: The Next Generation, to specifically counteract it with "The Bonding" and "Lower Decks." Indeed, the cartoon series "Lower Decks" kind of feels like it took some inspiration from Redshirts since its about the elaborate inner lives of the disposable faceless crew.
Still, bad writing jokes will never go out of style nor will the existential crisis a person would face if they discovered they were secondary to the plotline of someone else. Indeed, all of human life is pretty much us dealing with how we're just bit players to someone else's narrative. Politicians, corporate leaders, and even the ex who decided that we were best to be fired from the show of their own personal romantic comedy.
I would normally give this book a 5 out of 5. It is a Hugo-winning award after all and deservedly so. However, I feel this is a book that suffers from the fact that has way too many endings. The actual story ends roughly 75% way through the book and what follows is a series of vignettes following up on the events. These are done by bit players (ha!) in the story and expands on their story. Honestly, they don't really hold a candle to the main narrative and I wish they'd just been left out.
Redshirts is, for lack of a better description, a science fiction version of Rosencratz and Guildenstern are Dead. That play dealt with themes of bit characters, unnecessary deaths, being completely confused about what the hell was going on, and in-jokes that only someone who absolutely loves the subject being discussed (in that case: Hamlet) would get. There are much worse things to be compared to. I should note that I listened to the audiobook version and strongly recommend it for the fact it has the meta-casting of Will Wheaton as narrator.